Fix File Upload fatal (#25361) (#25393)

## Summary (fixes #25342)

There is a fatal when using the File Upload control in the image upload control.  While the fatal appears to originate within the File Upload control, it is actually [coming from](https://github.com/clintandrewhall/kibana/blob/file-upload/x-pack/plugins/canvas/public/components/loading/index.js#L15) `Loading`, which attempts to use `react-redux` to change the `backgroundColor` of the loading indicator based on the page.

I couldn't find an instance where we use this Loading indicator outside of a redux context, so I've opted to simply use the control directly.  @rashidkpc agreed this would be a good fix for the moment.

In addition, I fixed a props error coming from the data-url field that made it read-only, so I fixed that, too, as well as a formatting issue with the Set button.

## Test Plan

Added a `setTimeout` call to simulate load.  Toggled between data url and literal URL.

![screen shot 2018-11-07 at 6 47 32 pm](https://user-images.githubusercontent.com/297604/48171133-20ff4a80-e2c0-11e8-9b9f-71818801744a.png)

- Before
![screen shot 2018-11-07 at 7 11 48 pm](https://user-images.githubusercontent.com/297604/48171320-09749180-e2c1-11e8-8a2a-f87d564944c1.png)
- After
![screen shot 2018-11-07 at 6 57 17 pm](https://user-images.githubusercontent.com/297604/48171134-20ff4a80-e2c0-11e8-9bec-f129cfbfaa18.png)
- Fixed Warning
![screen shot 2018-11-07 at 7 11 57 pm](https://user-images.githubusercontent.com/297604/48171334-142f2680-e2c1-11e8-8ab6-ee06e6231e6b.png)

### Checklist

Use ~~strikethroughs~~ to remove checklist items you don't feel are applicable to this PR.

- [x] This was checked for cross-browser compatibility, [including a check against IE11](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility)
- [x] Any text added follows [EUI's writing guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses sentence case text and includes [i18n support](https://github.com/elastic/kibana/blob/master/packages/kbn-i18n/README.md)
~~- [ ] [Documentation](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#writing-documentation) was added for features that require explanation or tutorials~~
~~- [ ] [Unit or functional tests](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility) were updated or added to match the most common scenarios~~
~~- [ ] This was checked for [keyboard-only and screenreader accessibility](https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Accessibility#Accessibility_testing_checklist)~~

### For maintainers

~~- [] This was checked for breaking API changes and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)~~
~~- [ ] This includes a feature addition or change that requires a release note and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)~~

x-pack/plugins/canvas/canvas_plugin_src/uis/arguments/image_upload/index.js

@@ -7,6 +7,7 @@
 import React from 'react';
 import PropTypes from 'prop-types';
 import {
+  EuiForm,
   EuiFlexGroup,
   EuiFlexItem,
   EuiImage,
@@ -15,7 +16,10 @@ import {
   EuiButton,
   EuiFieldText,
 } from '@elastic/eui';
-import { Loading } from '../../../../public/components/loading';
+
+// TODO: (clintandrewhall) This is a quick fix for #25342 -- we should figure out how to use the overall component.
+import { Loading } from '../../../../public/components/loading/loading';
+
 import { FileUpload } from '../../../../public/components/file_upload';
 import { elasticOutline } from '../../../lib/elastic_outline';
 import { resolveFromArgs } from '../../../../common/lib/resolve_dataurl';
@@ -123,18 +127,19 @@ class ImageUpload extends React.Component {
     }
 
     const pasteImageUrl = urlTypeSrc ? (
-      <form onSubmit={this.setSrcUrl} className="eui-textRight">
+      <EuiForm onSubmit={this.setSrcUrl} className="eui-textRight">
         <EuiFieldText
           compressed
-          value={this.state.url}
+          defaultValue={this.state.url}
           inputRef={ref => (this.inputRefs.srcUrlText = ref)}
           placeholder="Image URL"
           aria-label="Image URL"
         />
+        <EuiSpacer size="m" />
         <EuiButton type="submit" size="s" onClick={this.setSrcUrl}>
           Set
         </EuiButton>
-      </form>
+      </EuiForm>
     ) : null;
 
     const shouldPreview =