Add section based signup method

[felixrindt]

Already fixes #11 and fixes #12 and also fixes #13.

[coveralls]

Coverage increased (+2.7%) to 76.773% when pulling d079c4b on section-based-signup into 3a7384a on main.

[jeriox]

If the user is not qualified for any sections, the signup button is disabled as expected. When accessing the signup action view directly via URL, one is presented with the following. Pressing "anfrage" loops back to the same page.

[jeriox]

there are no error messages when the planner does not select a section and tries to save

[jeriox]

If the user is not qualified for any sections, the signup button is disabled as expected. When accessing the signup action view directly via URL, one is presented with the following. Pressing "anfrage" loops back to the same page.

same thing applies for visiting the page while already being signed up. one can select a section and request, then a error message appears that one is already signed up

[jeriox]

This is really amazing! Only small detail that is missing IMO:
the user cannot see what sections are available prior to clicking the request button and being redirected.
three options:

• we do nothing about that for now
• the list of sections is rendered additionally (e.g. as part of the key-value pairs of the shift configuration)
• we render a list of sections where each section contains the users that have been confirmed for it and its properties (how many people are needed) for the shift state part of the shift box.

I think that option 3 is the way to go for the long term, would be fine with 1 or 2 for now

[jeriox]

please also have a look at the comments outside of the review :)

[jeriox]

Scenario: A participation has been confirmed (and therefore a section has been selected) and the user declines the participation afterwards. Once the user requests a participation again the request is displayed with the section he once had even if users are not allowed to pick a section.

[jeriox]

@felixrindt the user preference gets set to null once the participation is confirmed