Fix the merge notification email not being sent

The 'author_id_of_changes' attribute is not persisted in the database.
As we retrieve the merge request from the DB just before sending the
email, this attribute was always nil.

Also there was no tests for the merge notification code - tests have
been added.

Fix #6605
Add a rake task that drops all tables
Drop all tables before restoring a PostgreSQL DB

Invoking 'db:schema:load' turned out to be a bad idea: when downgrading
an existing GitLab installation, the schema of the newer version would
be preserved when trying to import the old version.

Conflicts:
	CHANGELOG
Update CHANGELOG for 6.7.2
Use current_user for Notification#merge_mr

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Remove modernizr gem.
Do not require modernizr.

Conflicts:
	app/assets/javascripts/main.js.coffee
 Merge branch 'patch/6-7-2' into '6-7-stable'

6.7.3 patch release
Bump version to 6.7.3, update changelog.
Fix avatars in the network graph for relative-url setups.
Merge pull request #6720 from thmo/6-7-stable

Fix avatars in the network graph for relative-url setups.
Revert "Merge pull request #6720 from thmo/6-7-stable"

This reverts commit e6707f6f278352d7ca63d9c0a3a3780e6c97be42, reversing
changes made to f88d30fac86b317248dbb47e01b050d9c1edbadf.
Don't send an email for "mentioned in" notes

Currently, an email is sent every time a mentionable is referenced
by an issue, a commit or a merge request: if I comment "This MR is
related to #5", watchers get one email for the comment, and another
one stating "Issue #5 was mentioned by issue #13".

This is annoying — but the biggest issue is when pushing an existing
branch. Every issue referenced by commit messages in this branch will
get a new mention (which is fine), and dozens of emails will be sent
for all these new mentions (which is not).

This commit fixes the spam by avoiding to send an email when a new
mention is created. In most cases the email notification for the
mentioner is enough.
Version to 6.7.4.
Fix emails on push service when a single commit is pushed

Conflicts:
	CHANGELOG
Version 6.7.5.
Add .pkgr.yml file for automated packaging on https://pkgr.io
Add rake task to install or upgrade gitlab-shell installation.