Clarify that users need to log in through an app

[RubenVerborgh]

We currently have 2 inbound links to the login page, but that page can only be used as part of an OIDC flow.

I would suggest to keep the login links (people will be looking for them), but then provide an explanation on the rendered login page.

@joachimvh Can we detect that we're not part of an OIDC flow in that handler?

[joachimvh]

@joachimvh Can we detect that we're not part of an OIDC flow in that handler?

There might be a cleaner way, but we can do the following call:
https://github.com/solid/community-server/blob/6312a178ae546a409fa640d461ed5a22e57cc080/src/identity/interaction/SessionHttpHandler.ts#L18

I'm pretty sure that one errors if we're not in the OIDC flow (or if there is something wrong with the session).

Would have to double-check though.

[joachimvh]

Some clarification has been added, but we want to add a more extensive error page which depends on the IDP refactor, so keeping this open until then https://github.com/solid/community-server/blob/f71f8683fc0f4e40de2e1c64547397f32c0b6472/src/identity/IdentityProviderHttpHandler.ts#L190-L194

[joachimvh]

Solved in #921, part of 2.0.0 release