reminder: add send to speaker option

[Naveenaidu]

closes #4958

Screenshot of the option:

TODO:

• Apply migrations
• Add Changelog entry
• Fetch the speakers and add them to all_recepients
• Format the code and apply the suggestion from pylint

[ThiefMaster]

Thanks! Please also add:

• a changelog entry (for 3.0rc2 - you need to add the "Improvements" section - please position it in the same way as in other versions) referencing the issue+PR
• the alembic revision (indico db migrate -m 'Add reminder send_to_speakers column') to create the column in an existing DB (see e.g. here for an example how to create a non-nullable column with its proper default value)

[Naveenaidu]

Thanks for reviewing @ThiefMaster ^^

I had a few questions regarding this. I'm still learning the codebase so please bear with me if the questions sound very naive :3

1. Should the send_to_speaker option be visible for all the Events? If yes, who would be the speakers in the meeting event?
2. To get the speakers for the lecture event, which part of the codebase should I look into?
3. For the conference event. Could you point me to the documentation if available to know who is considered as the Contributors and SubContributors. The terms are a little confusing to me. Also again which part of the codebase should I look into for them? Is it the contribution.py, subcontribution.py or the person.py files? I am still trying to figure out the correlation between them

I'm kinda thinking that I would definitely have to issue a db query to get the speakers. I am just not sure from what table I need to so that ^^'

[ThiefMaster]

I'd consider anyone who's a speaker anywhere in the event, including chairpersons (technically meeting/conference chairpersons are the same as lecture speakers). Probably it's a good idea to skip anything but those on the event in case the event type is lecture, just to avoid any weird surprises if someone changes event types.

For the event itself you can just iterate over the people in event.person_links.

For contributions etc. you need a query like this:

contrib_speakers = (
    ContributionPersonLink.query
    .filter(
        ContributionPersonLink.is_speaker,
        ContributionPersonLink.contribution.has(is_deleted=False, event=event)
    )
    .all()
)

Likewise for the other types. subcontributions need some more nesting, something like this (untested) should work:

subcontrib_speakers = (
    SubContributionPersonLink.query
    .filter(
        SubContributionPersonLink.is_speaker,
        SubContributionPersonLink.subcontribution.has(
            db.and_(
                ~SubContribution.is_deleted,
                SubContribution.contribution.has(is_deleted=False, event=event)
            )
        )
    )
    .all()
)