Tweak pipeline triggering settings

Tweak pipeline triggering settings

In an attempt to ensure that:

  - all important changes to repository contents are tested,
  - pipelines are not automatically created for every single push,
  - some flexibility is allowed for corner cases,

change pipeline triggering settings so that:

  - full build & test pipelines are only automatically created for merge
    requests and tags (both for creation and updates),

  - pipelines for other repository changes (e.g. pushes to arbitrary
    branches) can only be created manually, using GitLab's web
    interface,

  - merging a merge request only causes jobs pushing the updated ARM to
    GitLab Pages to be run (as semi-linear Git history is enforced and
    thus testing a MR is identical to testing the target branch
    post-merge in terms of code),

  - repository synchronization does not trigger duplicate pipelines in
    projects which are set as mirroring targets.

(cherry picked from commit 1c8c1815e4bf65e8f367c1f3141adf70a20a4295)