merge hashtag dialogs into one (#4861)

A hashtag picker dialog was implemented twice, with slight differences.
Now there is only one
- with hashtag validation - no more api errors when following an invalid
one
- The dialog can now be closed with the keyboard, for extra fast hashtag
selection
- with autocomplete

I also added a new snackbar when following a hashtag was succesfull.

Although I'm not sure about the auto complete, it can be very annoying
as the drop down covers the buttons. I found no way to make it size to
its content: https://chaos.social/@ConnyDuck/113803457147888844

Should we get rid of it?

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

@@ -379,7 +379,7 @@
     <string name="delete_scheduled_post_warning">Dileu\'r neges wedi\'u hamserlennu hon\?</string>
     <string name="description_post_media_no_description_placeholder">Dim disgrifiad</string>
     <string name="hint_list_name">Enw\'r rhestr</string>
-    <string name="edit_hashtag_hint">Hashnod heb #</string>
+    <string name="pick_hashtag_hint">Hashnod heb #</string>
     <string name="report_remote_instance">Anfon ymlaen at %1$s</string>
     <string name="report_description_remote_instance">Daw\'r cyfrif o weinydd arall. Ydych chi am anfon copi dienw o\'r adroddiad i\'r gweinydd hwnnw hefyd\?</string>
     <string name="duration_1_hour">Awr</string>