Enable Gitlab CI

Lint and build the cern-auth-proxy Helm chart with Gitlab CI, then push it to Harbor registry.

TODO:

• rebase after !1 (merged) goes in.
• create harbor robot account and add secret variables

Created robot robot-paas-tools+cern-auth-proxy with the following permissions:

• Push Artifact
• Pull Artifact
• Read Helm Chart
• Create Helm Chart Version

Fixes #1 (closed)