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-gd/strings.xml

@@ -171,7 +171,7 @@
     <string name="notifications_clear">Sguab às</string>
     <string name="list">Liosta</string>
     <string name="select_list_title">Tagh liosta</string>
-    <string name="edit_hashtag_hint">Taga hais gun #</string>
+    <string name="pick_hashtag_hint">Taga hais gun #</string>
     <string name="add_hashtag_title">Cuir taga hais ris</string>
     <string name="hint_list_name">Ainm na liosta</string>
     <string name="description_poll">Cunntas-bheachd le roghainnean: %1$s, %2$s, %3$s, %4$s; %5$s</string>