Regroup API Signin Routes + Transfer UI conditions to API

[fmigneault]

TODO:

• make a single route /singin for any type (local ziggurat or external provider)
  (handling of external redirects to be done by UI)
• let the API do the provider type switch, not the UI
• update REST API docs in repo + swaggerhub

Checks:

• ensure that external signin providers work (redirect to their login page + logged in after)
• ensure that cookies are properly saved/cleared from sigin/signout via API & UI
• ensure JSON & form/data compatibility for /signin (ref API requests with POST not accepting JSON format #50)

References / track source:

• blame login change
• diff signin external

Extra:

• modify credentials message while editing UI (issue Make invalid credentials less informative #59)

[fmigneault]

[fmigneault]

Relates to issue #68 and OGC-20