Expose the header matches in the REST API

This branch exposes a mailing list's header matches in the REST API.

It is composed of 3 commits that are best understood in sequence:

• commit 3e69044a turns the former HeaderMatchSet into a HeaderMatchList because the order in which the header matches are processed by the rule is significant. For this reason, the HeaderMatch model classes have gained an index attribute that allows sorting and re-ordering. HeaderMatchList does not entirely behave like a list because we want a better API for append and insert.
• commit 432b07c5 changes the newly introduced index attribute into a property where the setter takes care of re-ordering the list of header matches.
• commit 703fc135 renames the HeaderMatch.chain attribute to action and coerces it to an Action enum. This is what this field is intended for, and allows validation of what the admin can set through the REST API.
• commit e1950ac8 adds the REST API endpoints and documentation.

Fixes: #192 (closed)