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-lv/strings.xml

@@ -531,7 +531,7 @@
     <string name="action_post_failed">Augšupielāde neizdevās</string>
     <string name="action_post_failed_show_drafts">Rādīt melnrakstus</string>
     <string name="action_post_failed_do_nothing">Aizvākt</string>
-    <string name="edit_hashtag_hint">Tēmturis bez #</string>
+    <string name="pick_hashtag_hint">Tēmturis bez #</string>
     <string name="status_edit_info">%1$s laboja</string>
     <string name="action_browser_login">Pieslēgties ar pārlūku</string>
     <string name="description_login">Strādā vairumā gadījumu. Dati netiek nopludināti uz citām lietotnēm.</string>