New branch button on issues

Creates a button which creates a branch for you, referencing the original issue in the branch name. When creating the MR the text `Closes #iid` is appended to the description field.

The button; with styling

![Screenshot_from_2016-02-13_17-28-54](/uploads/9595fbfef88c10a499829beaa3db11c1/Screenshot_from_2016-02-13_17-28-54.png)

Links to the related branches on the issue

![Screenshot_from_2016-02-13_17-31-48](/uploads/ee7a78754eb5e2389f8671771bb59af9/Screenshot_from_2016-02-13_17-31-48.png)

Styled like the MR links

![Screenshot_from_2016-02-13_17-31-53](/uploads/c60130333a650a16f51b014aeb1458c3/Screenshot_from_2016-02-13_17-31-53.png)


Please provide input on the following; the CI repo on the GDK had the following happening with the current way I implemented the matching:

![Screenshot_from_2016-02-14_08-57-19](/uploads/35e7a3687a0fa801aba1ad66305ab2ff/Screenshot_from_2016-02-14_08-57-19.png)

Closes #3886 

cc @DouweM 


See merge request !2808