Finetune node forms

Re-scoped and moved to reader module.

• Add autocomplete widget
• Add link widgets
• Move 'publishing' elements to separate group
• Add new image widget that makes uploading images pretty
• Add new file widget that makes uploading images pretty
• refactor AP outbox element in the reader context
• make exceptions for websub and webmention
• document the reader form mode in the UI too
• listen to form display to remove all the rest?
• add template overrides and setting to hide filter guidelines

Original idea:

Wild dream, inspired by Mastodon, cf screenshot.

It should be possible to create a single simple form with many options like:

• Post privacy
• Upload media
• Auto mention users
• Content warning (doesn't exist yet - created #3319422: Add content warning option for that)
• Smileys - low prio though :)

Currently I have many content types which have mapping of AP properties to their field. But it's kind of easy to just create one content type with all properties. When submitting this node then, the post privacy will take care of to/cc etc, all other properties (body, media will be mapped too).

• It would live in a dedicated form which can be called on a route, or inside something else (e.g. within the Reader module).
• So, It doesn't replace the addActivityPubOutboxFormElement element which exists, that can still live on.

Some thoughts:

• To use this form, you'd have to configure something in the back-end to tell which AP type config to use, in most cases, something like a 'Note'. Now, I even think it should be possible to map itself to multiple (e.g. reply could go into another content type, since that's how my content is organized on my site at the moment too) and submit will save into the right content type. Of course, in case that's hard to get right, we can always document how to do this programmatically.