Backport changes from master to 2.12 branch

This MR backports the changes done to the master branch to the 2.12 branch.

The MR was generated by the following command + an additional manual commit adding a comment to .gitlab-ci.yml in order to make the diff between the branches smaller.

git cherry-pick \
  210bbf33740c6e4ab2728a4eede01b67ed957d05 \
  265dbcf2163e5f3c5bd5e414a296615b4a33a696 \
  5206ed5c72d6434b36136ac5ae5e55002199b74b \
  f6baa4fccc17163556926a04fba2fa225b2df81d \
  e8e9e552de2b97a8f77248471a8273cf2c12e4b5

Backport of these MRs:

• !7 (merged): 210bbf33
• !13 (merged): 265dbcf2
• !19 (merged): 5206ed5c
• !20 (merged): f6baa4fc
• !23 (merged): e8e9e552

debian/watch

@@ -2,4 +2,4 @@ version=3
 # filenamemangle -> prepending icinga2 if only tag name
 # versionmangle  -> convert version addition to ~ version
 opts=filenamemangle=s/.+\/(?:icinga2-|v)([\d.]+(?:-.+)?).tar.gz/icinga2-$1.tar.gz/,versionmangle=s/^([\d.]+)-(.+)?/$1~$2/ \
-https://github.com/Icinga/icinga2/releases .*/(?:icinga2-|v)([\d.]+(?:-.+)?).tar.gz
+https://github.com/Icinga/icinga2/tags .*/(?:icinga2-|v)([\d.]+(?:-.+)?).tar.gz