Support running argus as subsite

[hmpf]

Instead of serving the frontend on "DOMAINNAME/" and the api on "DOMAINNAME/api/" it should be easy to reroot them, for instance frontend on "DOMAINNAME/examplefrontendurl/" and api on "DOMAINNAME/exampleapiurl/".

[hmpf]

Problems:

src/argus/base/templates/index.html

5: <a href="{{ frontend }}">Frontend</a>

src/argus/htmx/templates/registration/login.html

20: <a class="btn btn-primary" href="{{ backend.url }}">{{ backend.display_name }}</a>

[lunkwill42]

Instead of serving the frontend on "DOMAINNAME/" and the api on "DOMAINNAME/api/" it should be easy to reroot them, for instance frontend on "DOMAINNAME/examplefrontendurl/" and api on "DOMAINNAME/exampleapiurl/".

I don't understand the need for splitting the URL scheme in Argus this way - it has no bearing on our use case as far as I can see, it just makes the solution more complex than it needs to be. A single prefix for the whole of Argus should be sufficient, so using /argus/ as a prefix on https://nav.example.org/ would put all of Argus on https://nav.example.org/argus/, including its API at https://nav.example.org/argus/api/