Remove teacher-specific grey box from learn.code.org, and for teachers have a blue button that links to ne teacher dashboard.
canonical_hostname helper for dashboard
add canonical_hostname (same as the same in pegasus/website-ci) to dashboard
for teachers: link to teacher dashboard from studentmode homepage; redirect to teacher dashboard when signing in
Fix style on learn.code.orge that points to teacher dashboard.
fix prizes url
redirect old teacher dashboard urls to new teacher dashboard
ask for email instead of username in /join link
remove debugging print
Merge branch 'teacherupdate' of github.com:code-dot-org/dashboard into teacherupdate
Merge branch 'finished' into teacherupdate
Teacher signed into code.org gets new dashboard URL.
Merge pull request #1161 from code-dot-org/teacherupdate

WIP Teacherupdate -- replacing old teacher dashboard with new teacher dashboard