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>
 Merge branch 'patch/6-7-2' into '6-7-stable'

6.7.3 patch release
Bump version to 6.7.3, update changelog.