UI improvements

First steps towards a better interaction. I've added bootstrap and set a common style for most pages. In a further PR, we should address a better interaction (some pages could be modal windows, set redirects when a user completed an order, etc).

There is room for improvement, though:

- make header a common block, not copy-paste
- review login/signup/reminder/password_reset forms
- there is still some legacy pylons style around, should be deleted
- integrate placeholder into translation chain