Redirect to next page on sign in

[warrensearle]

Unauthenticated users should be redirected to their intended destination after they have authenticated.

Current behaviour
If a user tries to access a restricted/protected page they are taken to the login page.
If the user correctly logs in they are taken to the exercises list.

Expected behaviour
If a user tries to access a restricted/protected page they are taken to the login page.
If the user correctly logs in they are taken to the page they were trying to access.

Note: the Apply site was recently updated to behave this way, see this PR