Class comments emails: on changing a class, the subject line will contain "Re:".

See Issue #975. This helps differentiate between new class registrations and changes to existing classes.
Removes the auto_image/ directory

This is no longer needed, since we have generic templates now and no
longer generate colored icons.
Fixes tables in teacherreg class lists
Remove theme.css and theme-gen.css - outdated stylesheets
Remove siteisdown.html

We should potentially come up with a substitute that isn't site-specific,
maybe by rendering an error page to a certain location whenever the server
is started?
Remove errant </div> in teacherregcore mainpage.html
Merge pull request #1026 from learning-unlimited/issue975

Class comments emails: on changing a class, the subject line will contain "Re:".
Merge pull request #1077 from learning-unlimited/remove_auto_image

Removes the auto_image/ directory
Corrects docs for Two-Phase Student Reg
Merge pull request #1124 from learning-unlimited/doc-fix

Corrects docs for Two-Phase Student Reg
Fix bug in dbmail when extra headers are not set.
Merge pull request #1127 from learning-unlimited/dbmail_minorexceptionfix

Fix bug in dbmail when extra headers are not set.
Merge pull request #1078 from learning-unlimited/mit-fixes

Fixes tables in teacherreg class lists
Merge branch 'main' into mit-prod