autocomplete: fix usernames not being highlighted in @mentions.

Fix usernames not being highlighted when completing @mentions.

This also changes it so the autocomplete results don't include the '@'
in front of the name.

Minor breaking change to the /autocomplete.json API. Autocomplete
results for mentions now have the type `mention` instead of `user`.

app/logical/autocomplete_service.rb

@@ -362,7 +362,7 @@ class AutocompleteService
   # @return [Array<Hash>] the autocomplete results
   def autocomplete_mention(string)
     autocomplete_user(string).map do |result|
-      { **result, value: "@" + result[:value] }
+      { **result, type: "mention", value: "@" + result[:value] }
     end
   end