Doc for update from 5.3 to 5.4
Update installation.md

fixed branch-name (5-3 -> 5-4)
Merge pull request #4589 from lukas2511/patch-1

Update installation.md
Update VERSION to 5.4.0
Update references to 5-3-stable in install guide
Wotif-specific Gitlab customizations:

* Add user_id to user_created/user_destroyed system hooks.
* No-op project related emails, as they were too spammy.
* Small forgivable hack on auth code.
  Use userPrincipalName to work properly with CBO admin accounts.