Add support for destroying project milestones

### What does this MR do?

This MR adds a "Remove" button to the project milestones page and the milestone page itself.

### Why was this MR needed?

Because lots of people talked about needing to clean their toilets. :)

### What are the relevant issue numbers?

Closes https://github.com/gitlabhq/gitlabhq/issues/1504

### Screenshots

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/ef8c7a3ea1db7b37cccae3869ac4de0a/image.png)

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/33eb01c7bc30fb235de96db5efb8746d/image.png)

See merge request !980