Make sending email invites configurable

Introduce "SENTRY_ENABLE_INVITES" setting to enable/disable the
ability to send email invites.

And while we're touching the new.html template, change the submit
button text for sending an invite from "Save Changes" to
"Send Invite".
Merge pull request #1252 from jaysoffian/sentry_enable_invites

Make sending email invites configurable