Update header so that the entire GitLab logo is clickable

At the moment, there is a 6px gap either side of the GitLab logo that changes background colour on the hover, but is not a link.

This is an annoying bit of design.

This pull request should resolve that.

**Note**: These are fairly simple changes, but I have not tested them. Feel free to redo them properly, and use this as a bug report instead.