Add some colour and formatting of boxed things
welcome_page: Move out and add font declaration

This is not as it should be: refactoring and adding stuff in one. But
the addition was so small… Anyway, put the font declaration stuff from
welcome_page.clj in its own file. Add a font declaration for DejaVu
Sans. I'm probably not going to use this one, but it probably has the
same format as all other declarations of fonts from Font Squirrel.
Finish the styling draft

 - Using Flexbox, fix the footer.

 - Introduce Flexbox for the rest of the main layout. (I know it's not
   intended for large-scale layout, but everyone uses it for that,
   because Grid is not ready yet.)

 - Change the accent colour. Violet didn't really fit.

 - Make some other adjustments.

 - Fonts and favicon not yet.

Note that there are one or two dirty hacks in this. However, it's just a
draft and I'm considering using Bootstrap for the real system anyway.