feat(sidebar): Add frontend API that allows to register custom entries

[susnux]

• Resolves Improvements for Comments in Activity #278
• Replaces POC: Minimal example of actions API #1396

This adds a front end API that allows to register plugins on the Activity tab:

1. Register custom actions, this actions will be shown on top of the activity bar (e.g. a comments input)
2. Register custom stream elements, meaning custom entries in the activity stream (e.g. comments)
3. Register custom filter function for shown activities to filter out some

Why showing custom elements instead of the activity? Activities will be cleaned up while e.g. comments do not. So this allows e.g. comments to be shown within the activity sidebar tab.

[susnux]

The diff looks awful but in reality e.g. Activity.vue was just renamed to GenericActivity.vue and Activity.vue is now a wrapper...

[susnux]

/compile amend/

[susnux]

A screen cast of how it looks when comments use it can be found here:
nextcloud/server#41491

[susnux]

/compile amend/

[github-actions]

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