feat (#1730): Allows scheduling of pending accepted/pending confirmed talks

[margau]

This PR is a draft implementation for #1730, and allows scheduling (by creating slots) for talks that are not finally accepted/confirmed, but only pending.

This allows internal time planning for organizers while not yet having to confirm the talk.
Public visibility is not changed.

How has this been tested?

Manually tested it by moving the states forward & backward

Checklist

• I have added tests to cover my changes.
• I have updated the documentation.
• My change is listed in the doc/changelog.rst.

Open Points/RFC

This PR, at the current stage, is intended as an RFC. Open Points:

• in general, would you merge this feature? If yes, I'd clean/finish the PR.
• shall we add a config option to enable this behavior? On or off by default?
• I'm not so happy with the calls of update_talk_slots() - maybe this should be called at the end of every Submission Update to reduce complexity?

[rixx]

Hm, I'm not sure about this. We'd need at least a way to visually distinguish these two types of sessions in the schedule editor, because otherwise, how are organisers going to know which part of their schedule is actually already set and communicated to speakers (and, potentially, the public), and which part is entirely tentative?

Currently, it's pretty clear what's going on with the regular vs striped session blocks. Once we introduce a third state, we'd need to explain what's going on, probably, so we'd need to figure out some fitting UI for that.

[margau]

I've pushed a version (dotted border, clear text indicating the pending state) for all non-accepted or non-confirmed talks:

This is not relevant for talks already accepted but pending confirmation, which are shown as normal accepted talks.

What do you think about this visualization?

[rixx]

I'll make some small adjustments after the merge, but nothing to worry about. Thank you so much for your contribution!