Login: rework ui

[tkohr]

PR intends to refine the login UI based on the work done in MEL project.

It includes only styling part, no error handling or translation capabilities.

Might need rebase on https://github.com/georchestra/georchestra-gateway/pull/27.

I see that there are two static folders on different levels, not sure how they are used. Also is the index.html essential?

[jeanmi151]

@tkohr @emmdurin can we merge ?

[tkohr]

To me, it looks good. @emmdurin anything to add?

[emmdurin]

OK for me.