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/strings.xml

@@ -82,7 +82,7 @@
     <string name="title_edits">Edits</string>
 
     <string name="dialog_follow_hashtag_title">Follow hashtag</string>
-    <string name="dialog_follow_hashtag_hint">#hashtag</string>
+    <string name="follow_hashtag_success">Followed hashtag %1$s</string>
 
     <string name="post_username_format">\@%1$s</string>
     <string name="post_boosted_format">%1$s boosted</string>
@@ -616,7 +616,7 @@
     <string name="hint_list_name">List name</string>
 
     <string name="add_hashtag_title">Add hashtag</string>
-    <string name="edit_hashtag_hint">Hashtag without #</string>
+    <string name="pick_hashtag_hint">Hashtag without #</string>
     <string name="hashtags">Hashtags</string>
     <string name="select_list_title">Select list</string>
     <string name="select_list_manage">Manage lists</string>