Devise views

WIP #184: https://assemblymade.com/helpful/wips/184

Abstracted the devise views in the Bootstrap `.panel`-component. I think this will work well.

I went with the standalone page designs because that's what the Sign up / Sign in pages are too at the moment. I think modals could work better, so I already added a `.panel-fixed` class to make the panels act like modals.