viewing a nonexistent group should be a 404, not a 500

fixes Sentry: /dmt/dmt/group/351/
restore prominent weekly report button to home page

IMO, the weekly report needs to be made very prominent to encourage time tracking
and self-reflection.
Merge pull request #146 from thraxil/bugfix-sentry-351

viewing a nonexistent group should be a 404, not a 500
Merge pull request #147 from thraxil/restore-weekly-report-button

restore prominent weekly report button to home page
changed colors for the time status indicators
Merge pull request #148 from zmustapha/userhomepage

changed colors for the time status indicators
Added instruction link for Markdown syntax
Merge pull request #149 from zmustapha/userhomepage

Added instruction link for Markdown syntax, and other small fixes