Systematize names: some easy pickings

[hmpf]

Adapted from Uninett/argus-htmx-frontend#230

Whether the name of something is singular or plural is a bit of a mess. Django custom is singular for models, modelname_list in template names that handles more than one of model-name's instances.

These are the easiest pickings.

Bonus: renaming the template for the checkbox to select all incidents to a name that better reflects that.

What remains are:

• htmx/themes
• htmx/incidents
• htmx/destinations

Renaming htmx/incidents to htmx/incident will be the big one.

"preferences" stays plural since one instance of the Preferences model contains multiple preferences. (Maybe the Preferences-model could be renamed to "PreferenceNamespace").

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.19%. Comparing base (0e44800) to head (c69df7f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/argus/htmx/incidents/views.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1015   +/-   ##
=======================================
  Coverage   81.19%   81.19%           
=======================================
  Files         140      140           
  Lines        5074     5074           
=======================================
  Hits         4120     4120           
  Misses        954      954           

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[podliashanyk]

Have tested manually with HTMX frontend and nothing seems to break there

[sonarqubecloud]

Quality Gate passed

Issues
0 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]

htmx/destinations were renamed via #970.

[hmpf]

htmx/incidents is handled by #1098