Add GitLab CI

This MR introduces GitLab CI to the fem-overlay.

Pipeline features:

• overlay-cli checks
• repoman checks
• pkgcheck checks
• Ensure formatting rules for ebuilds (can be done some later time)
• try to emerge new or changed ebuilds (can be done some later time)
• occasional build of a random ebuild in the overlay (done by the Tinderbox service)

Before merging:

• Squash commits!

Fixes #16 (closed)