In this PR, we added functionality to hide the admin header for edit/new actions - https://github.com/discourse/discourse/pull/30175
To make it work properly, we have to rename `show` to `edit` which is also a more accurate name.
This commit converts the current chat plugin UI into the
new "show plugin" UI already followed by AI and Gamification.
In the process, I also:
* Made a dedicated /new route to create new webhooks
* Converted the webhook form to FormKit
* Made some fixes and improvements to the `AdminPluginConfigPage`, `AdminPageHeader`,
and `AdminPageSubheader` generic components, so more plugins can
adopt the UI guidelines too. This includes adding a header outlet so plugins
can add action buttons to the plugin show page header.
* Fixes the submit button loading state for FormKit (by Joffrey)
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Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>
