feat: Ability to remove inbox avatar (#2885)

* Delete inbox avatar

1) New API endpoint added for deleting inbox avatar.
2) Delete avatar button in the inbox settings page.

Co-authored-by: Sojan Jose <sojan@pepalo.com>
Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/policies/inbox_policy.rb

@@ -53,4 +53,8 @@ class InboxPolicy < ApplicationPolicy
   def set_agent_bot?
     @account_user.administrator?
   end
+
+  def avatar?
+    @account_user.administrator?
+  end
 end