Feature/improve header and navigation styles

[andresgutgon]

What

• [x] Header is fixed on top
• [x] Header is responsive
• [x] User options going in a dropdown
• [x] On menu item click collapse menu on small screen
• [x] Your groups on a dropdown on Header

Fixed

Responsive

[enricostano]

OMG 👏 @andresgutgon !!!