Complete HTMX destinations page

[stveit]

#970 was merged, but with some remaining problems. The things that need to be fixed are:

• HTMX destination forms enforces unique label across medias #1075
• Error text on destinations HTMX page should be red #1076
• Add vertical gap between collapse element and create form on HTMX destinations page #1080
• Add padding/spacing around collapse and create form
• Centre the content of destinations HTMX page #1079
• - HTMX suggestion: If we put id on the parent div (the one with class .collapse-content) and put a highlighted div (lines 7-10) into a separate file, then we can return that file in create and update endpoints. This logic combined with using hx-swap='beforeend' would make more granular swaps on create and update actions. (see comment for more context)
• - HTMX suggestion: If we put id on each form-div, we could use it as a value in hx-target here. This combined with hx-swap='delete' would lead to a more granular swap on delete action. (see comment for more context)