Add rudimentary timeslot support

[hmpf]

It works but is almost indescribably fugly. No magic, to add another time recurrence it is necessary to "update" more than once.

As we plan to entirely hide the implementation of time recurrences, this is fine.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 47.94521% with 38 lines in your changes missing coverage. Please review.

Project coverage is 80.84%. Comparing base (f08319f) to head (acfd5c5).

Files with missing lines	Patch %	Lines
src/argus/htmx/timeslot/views.py	47.22%	38 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1087      +/-   ##
==========================================
- Coverage   81.29%   80.84%   -0.45%     
==========================================
  Files         141      142       +1     
  Lines        5089     5154      +65     
==========================================
+ Hits         4137     4167      +30     
- Misses        952      987      +35     

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[hmpf]

List of timeslots, currently:

[hmpf]

View time slot:

[hmpf]

Update timeslot (Create is almost identical, just missing the empty extra form at the bottom.)

Note the dark background in the input fields. I suspect our "argus"-theme cannot be used together with dark mode, there's something we've forgotten to set. This should not be fixed per input-field but globally, once.

[hmpf]

Finally, the delete-view:

[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[hmpf]

New screenshot of the update/create page after we found the right way to fix the input field backgrounds in #1089

[podliashanyk]

It is an absolute bare minimum timeslots page, with potentially some bugs (see comments below) and much follow-ups needed. The page is borderline un-understandable because of the UI (when I click on "Update" button somewhere in the middle, I am not immediately sure whether I will update the timeslot above or below f.e.), so some basic daisy-"cardification" plus minimal page organization with basic flexbox setup is high priority next step I would say. I suggest we make a meta issue and include this PR as one of the tasks there.

Do I understand correctly that some views seem redundant but are probably just a transitional step before we land on the page design (see comments below)? Also return values of most views will need to be changed in the future. F.e. we have redirects on timeslot delete action, while we need just to show the confirmation modal for it.

After a manual test this PR seems OK to me functionality wise (except what's mentioned in the comments below). Timeslots and recurrences are updated/created as expected (although the functionality is difficult to navigate in).

There will be HTML updates needed (adding h1, wrapping with fieldsets), but those belong to separate PRs.

[hmpf]

See #1094