[TASK] define 'schedule' table(s) and relevant classes

[aestallon]

Schedules is a special kind of document describing an approximately weekly business schedule.

It has some metadata (uploader, creation date, title, valid from, valid until, additional comments) and a list of ScheduleEvents: these consist of a date-time, a title, and an event type. The event type may be only a string for now, think about refining it to an enum.

Create the relevant database tables, repository class and service class