This change adds support retroactively updating display names in the new quote format when the user's name is changed. It happens through a background job that is triggered by a callback when a user is saved with a new name.
When we introduced the new quote format with full-name display name:
```
[quote="Ted Johansson, post:1, topic:2, username:ted"]
we overlooked the code responsible for rewriting quotes when a user's name is changed.
```
The functional part of this change adds support for the new quote format in the code that updates quotes when a user's username changes. See the test case in `spec/services/username_changer_spec.rb` for the details.
In addition, this change adds a regression test for PrettyText to cover the new quote format, and extracts the code responsible for rewriting raw and cooked quotes into its own `QuoteRewriter` class. The functionality of the latter is tested through the tests in `spec/services/username_changer_spec.rb`.
