OptaShift: add and remove tenants

See also
https://stackoverflow.com/questions/49724955/optpanner-roster-unable-to-add-tenant/49727902#49727902

Requirements:

• Next to the tenant combobox, we probably want a "gear icon" button to go to "the admin settings". Check with Matthew for a better UX idea.
• In the admin settings, we want a crud like screen for the tenants, with the ability to add a new tenant (or rename an existing one)
• When adding a new tenant, they must "initialize it", this implies:
  • choosing the "first date the schedule starts", aka the beginning of time: for example you might say, let's start "the 1th of next month rolling this out in production". We need a good variable name for this and I would expect it on RosterState.
  • publishNotice, defaulted to 7 (1 week)
  • firstDraftDate defaulted to beginning of time, it can never be less (use javax.validation to assure that)
    • Notice that if firstDraftDate is (or is close to) the beginning of time, the number of published days might be less than the publishNotice and that's fine.
    • As long is now() is before the publishDeadline, there is no need to warn the user that they are late to publish their schedule.
  • publishLength is 7, but that must be even read-only
  • rotationLength defaulted to 28 (so 4 weeks)
  • draftLength defaulted to 21