Follow:
* #22697
There are some bugs in #22697:
* https://github.com/go-gitea/gitea/pull/22697#issuecomment-1577957966
* the webauthn failure message is never shown and causes console error
* The `document.getElementById('register-button')` and
`document.getElementById('login-button')` is wrong
* there is no such element in code
* it causes JS error when a browser doesn't provide webauthn
* the end user can't see the real error message
These bugs are fixed in this PR.
Other changes:
* Use simple HTML/CSS layouts, no need to use too many `gt-` patches
* Make the webauthn page have correct "page-content" layout
* The "data-webauthn-error-msg" elements are only used to provide locale
texts, so move them into a single "gt-hidden", then no need to repeat a
lot of "gt-hidden" in code
* The `{{.CsrfTokenHtml}}` is a no-op because there is no form
* Many `hideElem('#webauthn-error')` in code is no-op because the
`webauthn-error` already has "gt-hidden" by default
* Make the tests for "URLEncodedBase64" really test with concrete cases.
Screenshots:
* Error message when webauthn fails (before, there is no error message):
<details>
</details>
* Error message when webauthn is unavailable
<details>
</details>
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