Add schedule timezone column

[jrauh01]

Add a column to store the timezone the schedule is created for.

Draft until the following PR is merged:

• Blocked by: Proper timezone handling for schedules icinga-notifications-web#369 (This schema change is solely needed for Notification Web, so no need to merge this earlier than the corresponding PR there. On the other hand, the schema change breaks Notifications Web without that PR as is couldn't insert rows anymore as it doesn't yet know of the required column.)

[oxzi]

Schema upgrade files are also required. Please consider some useful default for migration.

Please take a look at #324, where such files are already being created.

[nilmerg]

The default an be inferred from the first timeperiod_entry.timezone.

[jrauh01]

@nilmerg not sure if I understand this right, but the timeperiod_entry.timezone does not have a default.

Edit: Or do you mean to select the timezone value from the first timeperiod_entry that relates to the schedule and use the selected value as default for the migration?

[nilmerg]

select the timezone value from the first timeperiod_entry that relates to the schedule and use the selected value as default for the migration?

Exactly.

[julianbrost]

This PR is fine and can be merged any time the corresponding Notifications Web PR is ready. Though it probably shouldn't merged earlier as the added column is required without a default, hence without the corresponding change in Notifications Web, older version can't insert new rows anymore.

Thus, I've converted the PR to a draft for the moment (so there's a hint that it maybe shouldn't be merged immediately despite this approval) with a corresponding note in the PR description.