Regression tweet links to issue

/cc @job @marin @jacobvosmaer

Add image to backup task

Rename MergeRequest#lock! to #lock_mr!

As a warning from the StateMachine gem points out to us, there already
exists a method ActiveRecord#lock!. This change renames the `lock` and
`unlock` events defined on MergeRequest to `lock_mr` and `unlock_mr`.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Small changes

Rename home tabs

* Dashboard Home -> Activity
* Project Home -> Activity
* Group Home -> Activity
* Profile Home -> Profile
* Admin Home -> Dashboard

Related to #1351

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Commit log 25

Related to #1381

Improve markdwon parsing

Fix 500 error when try to use relative links in project issues if repository is empty

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Dash between description and readme link

Remove unused group filter

* dont try to build links for anchors like `#title-anchor`
* dont build relative url for project with empty repo
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

do not advertise gravatar in profile if disabled

Document the basics of working with Groups

Fix the PullReview tool link

Small contributions no rebase

Based on https://github.com/gitlabhq/gitlabhq/pull/7067#issuecomment-45060241

/cc @dzaporozhets