Find a solution for flutter_mentions

[gnunicorn]

We have several issue that seem to directly come from our usage of flutter_mentions. The dependency is clearly not maintained at this point and we need to decide what we want to do about it.

Most notably it seems to cause:

• Blob from text editor shows #1679
• https://github.com/acterglobal/a3-meta/issues/213
• https://github.com/acterglobal/a3-meta/issues/246
• Keyboard cover when uploading files and also after posting files #1790

[gnunicorn]

We are going to fork flutter tagger into our repo and start working from there replacing the existing mentions with our own implementation.

[gnunicorn]

so, experimented with TypeAhead as an alternative, also pretty nice. However we face a similar issue here: We have to load the entire list of userIDs and displayNames once and check for them, thus potentially being a lot slower in even displaying the first set of items. If we were to use a riverpod-ish system though, we could have first checks go to ref.watch(memberDisplayName(..)).valueOrNull and whenever the becomes available re-evaluate it. that'd actually be nicer.

[gnunicorn]

also as a reference point, this has some nice usage pattern: https://pub.dev/packages/multi_trigger_autocomplete

[gnunicorn]

but purely technically speaking, what do we need/care about at this point?

1. we want to notice certain characters being typed (for now @ does it)
2. if that happens, we want to have Flutter Overlay appear showing us options, matching the list to the input
3. when a user selects any of them, we replace the content of the textfield with the corresponding match

The list in 2 should probably just be a simple widget that we can then feed a riverpod consumer into so we can live update and don't have to wait for avatar data and stuff.

nice to have, follow up after:

• up and down arrows in the text field select items in the view, pressing enter selects it.
• render the entry in the box differentely (user-pill with avatar and display name instead of the regular content)
• other inputs we replace (e.g. emoji, and # to search rooms)