[#2689+2690] Improve help texts for notification settings

[pi-sigma]

The notification settings in the general site configuration are confusing, mainly because the relationship between enabling these options and the corresponding choices for the user is not obvious. For example, the help text for for notifications about expiring actions suggests that the user will be able to enable/disable these notifications in their profile (or when registering) directly, when in fact the user's choice is logically determined by their choice of notifications about plans).

The PR cleans this up by improving the help texts and rearranging the settings (both in the code and how they appear in the admin and in user-facing templates).

Taiga: https://taiga.maykinmedia.nl/project/open-inwoner/issue/2689, https://taiga.maykinmedia.nl/project/open-inwoner/issue/2690

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.19%. Comparing base (a430792) to head (be68a61).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1359   +/-   ##
========================================
  Coverage    95.19%   95.19%           
========================================
  Files         1002     1003    +1     
  Lines        37029    37033    +4     
========================================
+ Hits         35248    35252    +4     
  Misses        1781     1781           

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