Replace isOrgaTeam with may(orga), remove $_SESSION['client']['group']

Closes #883 (closed)

What does this MR do?

• remove isOrgaTeam
• remove isSiteAdmin
• remove some unused stuff (ApplicationControl->mode / $_SESSION['client']['group']
• rename methods/data in the vicinity for clarity

How confident are you it won't break things if deployed?

not very! this is old code and it's not always easy to reason about.

Checklist

• no functional change intended => no test
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.