Most directly useful for the migration to zulipchat.com.
Creates a new field in UserProfile to store the tos_version, as well as two
new settings TOS_VERSION and FIRST_TIME_TOS_TEMPLATE. We check for a version
mismatch between what the user has signed and the current
settings.TOS_VERSION whenever the user hits the home page, and redirect them
if needed.
Note that accounts_accept_terms.html and
zerver.views.accounts_accept_terms were unused before this commit
(they date from c327446537)
Rishi Gupta