[frontend] Force login buttons to appear if special URLs are used

[pfortuna]

People that used Google are seeing only Facebook.

[pfortuna]

We couldn't get the problem to manifest itself in staging. Nevertheless, we agreed in implementing this feature:
If the user enters the app using the normal path:
https://app.covidografia.pt/#/login
Then the login buttons are shown according to the loginmethod entry in the localstorage. This is exactly how it works now.

However, if the user uses this URL, then only the Facebook button should be shown, regardless of the localstorage entry:
https://app.covidografia.pt/#/login/facebook

If the user uses this URL, then only the Google button should be shown, regardless of the localstorage entry:
https://app.covidografia.pt/#/login/google