Merge when build succeeds

### What does this MR do?
Adds a button to a MR when the build/ci is running so it can be merged when/if the build is successfull.

### Are there points in the code the reviewer needs to double check?
English spelling and whether or not the grammer is correct.

### Why was this MR needed?
When you expect its all good, and don't want to revisit the current MR it can be accepted allready.

### What are the relevant issue numbers / Feature requests?
Fixes #2640 -- although `Merge if green` is replaced with `Merge when the build succeeds` the general idea is the same.

### Screenshots (if relevant)
![Screenshot_from_2015-12-07_10-34-39](/uploads/b90b558b6ad6c8266bc9c96120d86f96/Screenshot_from_2015-12-07_10-34-39.png)

![Screenshot_from_2015-11-02_17-26-56](/uploads/9f52f56e3c5e9ec63cb8f42978c92a52/Screenshot_from_2015-11-02_17-26-56.png)
![Screenshot_from_2015-11-02_17-27-03](/uploads/119fc18c9b15ff73dc38010ac5b6244a/Screenshot_from_2015-11-02_17-27-03.png)

![Screenshot_from_2015-12-03_10-08-14](/uploads/bad7b35c7d129981d43631877c958be0/Screenshot_from_2015-12-03_10-08-14.png)

![Screenshot_from_2015-11-02_17-27-16](/uploads/b616d750a16cc11ba72f2ca84213515e/Screenshot_from_2015-11-02_17-27-16.png)

### Further considerations
What if there are minor things needed solving, e.g. Rubocop, the current implementation will reset the approved status of the MR. It might be a consideration keep the approval for team member, or even guests. This would require an extra option in the Admin screen, though might add extra value.

#### TODO
- [x] Docs
- [x] Specs

/cc @DouweM @rspeicher

See merge request !1729