ComposeActivity refactor (#1541)

* Convert ComposeActivity to Kotlin

* More ComposeActivity cleanups

* Move ComposeActivity to it's own package

* Remove ComposeActivity.IntentBuilder

* Re-do part of the media downsizing/uploading

* Add sending of status to ViewModel, draft media descriptions

* Allow uploading video, update description after uploading

* Enable camera, enable upload cancelling

* Cleanup of ComposeActivity

* Extract CaptionDialog, extract ComposeActivity methods

* Fix handling of redrafted media

* Add initial state and media uploading out of Activity

* Change ComposeOptions.mentionedUsernames to be Set rather than List

We probably don't want repeated usernames when we are writing a post
and Set provides such guarantee for free plus it tells it to the
callers. The only disadvantage is lack of order but it shouldn't be a
problem.

* Add combineOptionalLiveData. Add docs.

It it useful for nullable LiveData's. I think we cannot differentiate
between value not being set and value being null so I just added the
variant without null check.

* Add poll support to Compose.

* cleanup code

* move more classes into compose package

* cleanup code

* fix button behavior

* add error handling for media upload

* add caching for instance data again

* merge develop

* fix scheduled toots

* delete unused string

* cleanup ComposeActivity

* fix restoring media from drafts

* make media upload code a little bit clearer

* cleanup autocomplete search code

* avoid duplicate object creation in SavedTootActivity

* perf: avoid unnecessary work when initializing ComposeActivity

* add license header to new files

* use small toot button on bigger displays

* fix ComposeActivityTest

* fix bad merge

* use Singles.zip instead of Single.zip

app/src/main/res/values-de/strings.xml

@@ -451,6 +451,5 @@
     <string name="action_access_scheduled_toot">Geplante Beiträge</string>
     <string name="action_schedule_toot">Plane Beitrag</string>
     <string name="action_reset_schedule">Zurücksetzen</string>
-    <string name="hint_configure_scheduled_toot">Drücke hier, um den geplanten Beitrag zu konfigurieren.</string>
     <string name="abbreviated_in_years">Dies sind Zeitstempel für Status. Beispiele: \"16s\" oder \"2t\".</string>
     </resources>