Introduce topAside block in the sidebar

This block goes above the body block when in mobile mode. This is to be
used sparingly, but seems to work well with the welcome message.
Remove homepage-layout template and use a variable to set sidebar width.
Merge branch 'update-homepage' into update-homepage-with-open-cfps
Smaller margin above a sidebar section.
Merge branch 'update-homepage' into update-homepage-with-open-cfps