Streamline posting to Lemmy

[TheMemeSniper]

Is your feature request related to a problem? Please describe.
posting to Lemmy communities requires a specific format (as described in LemmyNet/lemmy#2651)

Describe the solution you'd like
add a new drop-down menu to select post template fields. both extensible for other instance types (e.g. PeerTube and Pixelfed) and solves this solution cleanly. i.e.:

Microblog template:

[Text box]

Thread template

[Title]

[Link (optional)]

[Lemmy community]

Describe alternatives you've considered
there could be alternatives like detecting when you're trying to post to a lemmy community and formatting the post automatically, but i think the dropdown solution would be cleaner. either one would work, the dropdown may understandably be hard to implement.

Additional context
none