Avatars Refactoring

This PR does 3 things that are all slightly related:

1. Allow custom avatar handling via the new `avatar_url` option. Removed `avatar_default` option in favour of setting the default directly via `avatar_url`.
2. Remove `thredded_posts.user_email` column, adding robustness with regards to user email change. Avatars are no longer shown for `NullUser`.
3. Duplicate code from `(Private)UserTopicDecorator` extracted to `BaseUserTopicDecorator`. I usually prefer composition over inheritance, but followed the pattern for the other decorators here.

:tada: 
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It also has 2 tiny unrelated changes: removes spec/dummy/db/schema.rb, and adds `.includes(:postable)` to `TopicsController#index` (not so sure about this one but [Bullet](https://github.com/flyerhzm/bullet) was warning me).