* style pages which were not adapted yet
* streamline design (e.g. use the same forms everywhere)
* use less colors
* various fixes
* re-add token generation menu entry
* fix signup page to not show menu

let's use @birger's design language!

the idea of this patch is to use the same (default bootstrap layout)
navbar on all devices.

unfortunately for a vertical version of the default navbar we still
need two copies of the navbar. once outside of the grid for small
screens, to ensure that the navbar is on top of the content. And
a second time inside of the grid in a flex-column to the left.

also refactor the menu code to have a more declarative description
of the menu, that we could move the the config of the app, or even
automatically generate from the routes.

This commit makes users mobile friendly. To not have to maintain two
menus, the menu was exported to a _menu.html.erb and is included as
part of a navbar that is only visible on small devices and contains
a hamburger dropdown menu. The menu is also as a side menu for devices
that have bigger screens.
The main navigation items (username and logout) as well as the language
menu were integrated in the menu, so that we don't have to maintain
3 menus (which we would have to somehow display on mobile).
The 'card' container was replaced, because with the elements in the
card-title integrated in the menu and without a border it did not
make sense anymore.

the library knows better...

The TfaController now routes requests there instead of the
welcome page.

Use bootstrap cards for the two main 2FA options and use a table
for listing the existing 2fa setups

The controller now redirects back to the recovery email page after
the form was submitted.
The recovery_email_help_crypt text is not directly related to the
recovery email functionality so I removed it.

... and display the corresponding help using a danger alert.

... and use a bootstrap alert to display the success message

* Add the minified bootstrap css to the stylesheets
* Add the logo to the application.css
* Remove the topbar and replace it with a simple header
  containing the logo and the language selector
* Put the account settings into a bootstrap card component