Rename/reorder `foundation-5` and `custom_crispy_templates` folders in templates

[johannaengland]

The folders web/templates/foundation-5 and web/templates/custom_crispy_templates contain both templates that are build upon for form rendering and have been adapted for #2794. Maybe these folders could be merged and then renamed to something that does not refer to foundation/crispy-forms.

The file https://github.com/Uninett/nav/blob/3034cd50905721824c2df43710ca894d5d1a85ad/python/nav/web/templates/foundation-5/README.txt#L1also references crispy still and should be amended.

This was first discussed in #3211.