Sort and deduplicate username blacklist

Also convert to a plaintext newline delimited file -- there's nothing
gained from using YAML here at the moment.
Validate at registration that username is not blacklisted

This adds a validation rule in the registration schema that prevents
users from registering with blacklisted usernames.
Ship icomoon CSS in extension. Fixes #1572.
Lazy-load username blacklist

By lazy loading the blacklist applications can include the h package
and subsequently use the `config.override_assets` API to change the
blacklist file before the first request.
Merge pull request #1571 from hypothesis/username-blacklist

Username blacklist
Fix race in chrome injections

Close #1568
Pin sass install to work around sass/sass#1476

sass==3.4.6 has introduced a regression in the parsing of
pseudo-selectors such as

    :not(:focus)

which breaks our build. We need to temporarily pin to 3.4.5 to avoid
spurious build errors all over the place.
Add more usernames to blacklist

I noticed that root and postmaster were missing, one thing led to
another, and I added a bunch of names from this list:

    https://programmers.stackexchange.com/questions/115425/is-there-a-list-of-common-usernames-to-reserve-in-a-new-system
Unblacklist a few names

As @tilgovi points out, "nick" probably shouldn't be blacklisted. (An
honest mistake, guvnor! I promise!)

Also, java{,script}, ruby, php.
Merge pull request #1576 from hypothesis/username-blacklist

Add more usernames to blacklist
Move notification to a separate directory
Add includeme()
DeOOP Notification templates

ReplyTemplate and NotifierTemplates were semi-classes with only a bunch of
static functions collected under a class. Separating the templates in module level
makes these abominations obsolete.
Create reply-notification for newly registered users
Move domain_mailer into notifications
Notifications tab stub
Fix db write
Fetch subscriptions list from backend
Display subscriptions
Update notfication status