Helpers: Allow members to replace each other on tasks

[LajosCseppento]

Would make a bit of admin work go away.

User story:

• Member Joe wants to find a replacement for a task where he cannot contribute anymore
• Joe on the task view can press a button to start looking for replacement:
  • It should have a confirmation dialog
• After Joe confirms that he is looking for a replacement, he posts the task to the WhatsApp chat
• Alice sees it and wants to replace Joe. On the task view she can press a button to replace Joe
  • It should have a confirmation dialog
• Joe cannot replace himself, but have a different button confirming that he is available and does not need a replacement anymore

Notifications:

• Sent when the replacement takes place (to all past and future participants)
• Not sent when someone looks for a replacement or cancels looking for a replacement

Replacements can only happen for tasks which are:

• in the future, have not started yet
• are not marked as done
• are not validated

Replacements are possible for both shifts and deadline tasks.

[LajosCseppento]

Dilemma:

• should we have replacements managed on the task view
• should we managed the on a separate page, so all members can have an overview of all replacements

DB needs:

• either a separate table storing replacement posts
• or flags for captain and helpers that they are looking for a replacement

Backend needs:

• API endpoint to start looking for a replacement
• API endpoint to stop looking for a replacement
• API endpoint to "apply" a replacement

[LajosCseppento]

Extra details:

• when a task is marked as done or validated, replacements cannot happen anymore and all replacement requests on the task are deleted
• do not forget audit logging

[LajosCseppento]

See also #107 - do that one first and elaborate whether this more complex solution is worth the effort