Remove ActionText::Attachable from Tag

Tag was never actually embedded as an action-text-attachment in
persisted rich text. It was only used in the Fizzy Do command prompt
input which was not stored. The data transfer tests happened to use Tag
as a convenient fixture — switch them to User, which is genuinely
attachable via mentions.

app/models/tag.rb

@@ -1,5 +1,5 @@
 class Tag < ApplicationRecord
-  include ActionText::Attachable, Filterable
+  include Filterable
 
   belongs_to :account, default: -> { Current.account }
   has_many :taggings, dependent: :destroy