fix: Make oAuth paths more consistent and improve authentication client

[daffl]

This pull request makes the oAuth callback paths consistent. It uses the redirects discussed in simov/grant#116 to make things more straightforward and backwards compatible. Also changes the base path to /oauth as previously discussed with @KidkArolis (it can be made backwards compatible by setting authentication.oauth.defaults.path to /auth).

The relevant oAuth paths now are:

• http(s)://<host>/oauth/<provider> to initiate the oAuth flow
• http(s)://<host>/oauth/<provider>/callback as the callback path that should be set in the oAuth application settings
• http(s)://<host>/oauth/<provider>/authenticate as the internal redirect

[KidkArolis]

Looks good. Are you gonna be adding backwards compatible redirects?

[daffl]

I tried but couldn't get it to work. oAuth always errors when I try to redirect from a backwards compatible callback path. At the moment it looks like this is only possible if my PR in Grant gets accepted. I might submit it again to discuss more.

[daffl]

@KidkArolis Allright, got some help and managed to make the paths backwards compatible (updated in the description).