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changelog.d/+singularize-incidents.changed.md

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+Htmx: Customizers beware: Major refactor in src/argus/htmx/incident(s) and
+src/argus/htmx/templates/htmx/incident(s).
+
+* All directories named "incidents" was changed to "incident".
+* The templates that defines columns in the incident list was moved to
+  `htmx/incident/cells/`.
+* The template for selecting sources in the filterbox was moved to
+  `htmx/incident/widgets/`.
+* Whenever there were plural view-names or url-names for incident-related views
+  they were made singular.