Deviating shift participation times

[felixrindt]

Fixes #66. Fixes #524.

functionality

• model
• signup logic & form
• changing the time after signing up
• refactor signup configuration to use a dedicated form class
• dispatch form
• disallow deviating shift times
• make sure confirmed participations are not set back to required if only e.g. the comment changed
• Fix code TODOs
• Fix github comments
• Implement/discard comments on Allow participation times to deviate from shift times #66
  • esp. check that I can register with deviating time if the default time would be blocked by conflicting participations

fixing other places

• collision detection
• working hours count
• integrate into section based signup view
• check behavior with coupled signup, disallow deviation or don't copy rendering

user-informative changes

• update inline participation card
• event detail page
• update ical export
• update index page
• trigger notifications for changed times for confirmed participations
• update notification text

Move to issue when closing the PR

UX

• make the signup form UX nicer -> Improve Participation Form UX #617
  • maybe have a nicer widget that puts the date in a less important position
    • actually automatically determining the date might be the best solution. We just pick the date so the resulting datetime is closest to the initial value.
    • hide disabled widgets in signup view or explain that they are disabled.
• can we do better in form validation as suggested in the bootstrap docs? I don't think we should be moving all validation to the client side, but we could migrate the vanilla JS validation to be styled bootstrappy! -> Bootstrappy Form Validation #618

refactoring

-> #619

• move PDF export functionality to PDF exporter class
• consider refactoring Mixin based composition of signup methods (e.g. mixing Sections and Approval workflows)

Consequences for participant participation manipulation

This is probably very low priority, but we could build consequences to alter participation state. Although we would probably want to display them in the disposition view etc. -> #620

• Consquences for declining when self-decline is not allowed.
• Consequences for individual times when providing individual times is not allowed
• Allow participation times to deviate from shift times #66 (comment)

[coveralls]

Coverage increased (+0.2%) to 91.237% when pulling a7b0e33 on varying-shift-times into b76a7a9 on main.