Convert CI to Gitlab (was ACTS-350)

Original author Moritz Kiehn @msmk

Switch the ACTS continous integration to the builtin Gitlab CI. This would allow us to use the same LCG environment on lxplus, ACTS CI, and framwork CI for consistent results.

Edited Mar 27, 2019 by Moritz Kiehn

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information