Splash now extends base.html so there's no need for a marginless block.

For a little more consistency with the templates, the middcourses
logo appears on every page, included in the second level base.html
templates. The splash is the only page, besides these base.htmls
that extends the top level base.html, so it gets a special nav
button. Including the header here would result in two identical
middcourses logos right next to each other.

Change some additional formatting to start enforcing 80 char
line widths.