utils: improve extract_mentioned_users usefulness Previously, extract_mentioned_users performed a regex search, converted to a set to remove duplicates, converted that set back into a list, and sorted the list on username, ignoring case. Every single caller of the function then promptly took the carefully pruned and sorted result and put it back into a set, rendering the entire exercise pointless. In addition, every caller also resolved the usernames to database User objects. In this changeset, extract_mentioned_users is changed to return a set of database User objects directly. A new extract_mentioned_usernames function, which does only username extraction, is kept for the sole purpose of providing a more testable interface (no database needed). Bonus feature: The new extract_mentioned_users will prune non-existent users, as well as the DEFAULT user. This means it is no longer possible to @mention (and send notifications to) the DEFAULT user.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
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