read-only database support via decorator (#1441)

New @use_readonly_database decorator will set an override so that that
view will use the readonly DB.  New optional "readonly" database stanza.
Merge branch 'master' of github.com:Stanford-Online/class2go into sef-1441-db-router
database.py template for readonly db, fix typo (#1441)
Merge remote-tracking branch 'origin' into sef-1441-db-router
Merge remote-tracking branch 'origin' into sef-1441-db-router

Conflicts:
	main/courses/reports/generation/gen_in_line_reports.py
re-add @use_readonly_db to in-line report (#1441)
readonly db: add validations on save() and delete() for safety (#1441)

Picked some of the more heavily-trafficed models and added warppers
around the save and delete methods to just double-check that we're not
going to write to the read-only datbase from here.
fix delete guards, wrong prototypes (#1441)
Merge branch 'master' of github.com:Stanford-Online/class2go into sef-1441-db-router

Conflicts:
	main/courses/reports/generation/gen_in_line_reports.py
Merge pull request #2114 from Stanford-Online/sef-1441-db-router

Read-only database support (#1441)