added schedule change detection notification

[yeti107]

The bot now ...

• compares newly fetched schedules against previous schedule
• detects changes to existing sessions
• sends notification to the configured schedule updates channel

Method/Implementation

-Added ScheduleChange model for changed sessions
-Added schedule comparison logic in ProgrammeConnector
-Updated the schedule fetch task to send notifs when changes are detected
-Added tests for schedule comparison and notification handling

Ran tests/program_notifications and all 41 passed

[NMertsch]

Hi @yeti107 thank you for this PR. I hope this feedback is useful:

Missing functionality:

• I think your PR does not detect new or cancelled sessions.

Schedule change notifications:

• Schedule updates should mention if the title or speakers change.
• Schedule updates should always include the speaker, title, time and room. Users might be interested in "Hynek's talk" or "The talk right after lunch" without remembering the title.
• Room changes should be formatted for humans, not machines. Example: "Room changed: S1 -> S1, S2"
• Start time and duration should be treated as a single piece of information. Example: "Time changed: Thu 13:00 - 13:30 -> Fri 14:00 -> 14:30".
• The date and time formatting should be programmed explicitly, instead of relying on the server's locale.

Implementation

• ProgrammeConnector.fetch_schedule() should fetch the schedule. It should not return a list of schedule changes.
• Cog.fetch_schedule() should fetch the schedule. It should not send messages about schedule changes.
• I don't think this PR should modify mock_schedule.json.