Feature: Add filter to allow custom fediverse previews

[Menrath]

The current implementation of the Fediverse preview (#994), has a generic name but is still quite limited. Therefore I propose as suggested by @pfefferle to make it more flexible.

Two options come in to my mind:

1. Easy and straight forward: Apply a filter that lets change the preview template complety (based on the post-type).

2. Additional preview tabs within the fediverse preview: Within the preview, multiple preview tabs could also be possible, but it has a drawback that would add quite some complexity: e.g. an as:Event is rendered differently in Mastodon than a Note, same like Article. So the template would have to take Converted object types into account. So maybe here the preview tabs could be concrete Fediverse applications.

Reference:

         @Menrath nice idea! I will see how we can extend that a bit, to have a simple templating mechanism!?!

Originally posted by @pfefferle in #944 (comment)