Support Users Directly Uploading Profile Pictures.

* Adds a `picture` attribute to `User`.
* Preserves Gravatar as a fallback option.
* Now, you should use `user.profile_picturer` instead of `gravatar_url`
* TODO: Ensure `profile_picture` handles sizing right for uploaded images.

db/migrate/20200715034647_add_picture_to_users.rb

@@ -0,0 +1,5 @@
+class AddPictureToUsers < ActiveRecord::Migration[5.2]
+  def change
+    add_column :users, :picture, :string
+  end
+end