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-th/strings.xml

@@ -56,7 +56,7 @@
     <string name="list">รายการ</string>
     <string name="select_list_title">เลือกรายการ</string>
     <string name="hashtags">แฮชแท็ก</string>
-    <string name="edit_hashtag_hint">แฮชแท็กโดยไม่มี #</string>
+    <string name="pick_hashtag_hint">แฮชแท็กโดยไม่มี #</string>
     <string name="add_hashtag_title">เพิ่มแฮชแท็ก</string>
     <string name="hint_list_name">ชื่อรายการ</string>
     <string name="description_poll">โพลกับตัวเลือก: %1$s, %2$s, %3$s, %4$s; %5$s</string>