This PR includes the following refactors:
1. Moves the Index Pattern Api from _Stack Alerts_ to the _Server_ plugin of _Trigger Actions UI_. This fixes a potential bug where a user could disable the _Stack Alerts_ plugin and inadvertently break the UI of the _ES Index _ action type.
2. Extracts the UI components for _Index Threshold_ and _Geo Threshold_ from the _Trigger Actions UI_ plugin and moves them into _Stack Alerts_.
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