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-zh-rMO/strings.xml

@@ -358,7 +358,7 @@
         私信
     </string>
     <string name="hint_list_name">列表名</string>
-    <string name="edit_hashtag_hint">話題名（不含前面的 # 號）</string>
+    <string name="pick_hashtag_hint">話題名（不含前面的 # 號）</string>
     <string name="notifications_clear">清空</string>
     <string name="notifications_apply_filter">分類</string>
     <string name="filter_apply">應用</string>