Replaces "Create merge request" link with one to the MR when one exists

Fixes #3716

* Without MR:

![before-1](/uploads/abcb2eb5bb5927a83c2f1700528f852c/before-1.png)

* With MR:

![after-1](/uploads/9020b871e1aa08d3e5a45359336ef0f2/after-1.png)

* Without MR:

![before-2](/uploads/bb3bd1a74efc193ad31af68f1e04ef5f/before-2.png)

* With MR:

![after-2](/uploads/789d32aef12f39c605e665d0c4bc9fbe/after-2.png)

* Without MR:

![before-3](/uploads/a9aff7a11eb1069be24c37a5455be809/before-3.png)

* With MR:

![after-3](/uploads/48fe81ea50657fe5b2de0d25260d8e4b/after-3.png)

See merge request !2670