Make sure the posters fit within mobile viewports

The visitor can zoom in for more detail.
Let openweekPoster2013 use the same style as other recent posters

Although not really needed anymore, it looks a lot better now.
Use the Django URL name for the home link

This also fixes a small issue with posters: clicking the logo would
return to the poster, and not go to the homepage.
Merge pull request #225 from aykevl93/link-home

Use the Django URL name for the home link
Merge pull request #224 from aykevl93/style-openweekPoster2013

Let openweekPoster2013 use the same style as other recent posters
Merge pull request #223 from aykevl93/mobile-posters

Make sure the posters fit within mobile viewports
telefoonlijst: some updates
giedo/mailman: only create list if the group has a name
add bin/send-informacie-digest
member-graph: add argument