Time to first update of a feed when changing its schedule is passed as a number (in seconds) instead of a string.

Fixed unit test: instead of a default period of "1h", the starting update period for a newly created feed is expressed in seconds as "3600s".
Fixed unit test: when a missing feed update schedule is added back, the starting period is now "3600s" instead of "1h".
When fixing schedules, use the private method most public methods in ScheduleManager invoke, instead of doing it through a public method.
Fixed unit test. Added test to ensure that the maximum and minimum feed update intervals are enforced.
When fixing feed updates (FixSchedulesJob.perform) the next update is scheduled at the time when it should have been scheduled if the schedule needed no fix.

If that moment is in the past, the next update is scheduled for 1 second later (i.e. almost immediately).