Add locked_at to merge request

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Set/unset merge request locked_at timestamp after transition

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Automatically close merge requests that were locker for 1 day

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Add snapshot backup tips
Merge branch 'doc/snapshot_backups' into 'master'

Add snapshot backup tips

See merge request !1307
Merge branch 'mr-lock-problem' into 'master'

Fix merge request lock problem

Merge Request lock problem
MR can be stuck in lock state if satelitte timeout >= unicorn timeout.

Issue explanation:

* Person press Accept button
* unicorn worker starts, lock MR and trigger satellite
* satellite does merge but very long (30 seconds)
* unicorn timeout reaches earlier then satellite timeout
* unicorn kills worker because of timeout leaving MR in lock state.

Fix:

* set locked_at date
* if MR was locked for too long - close it automatically

- - -

Fixes #1674

See merge request !1306