Add UI for viewing and cancelling open Zulip invitations.

Lets administrators view a list of open(unconfirmed) invitations and
resend or revoke a chosen invitation.

There are a few changes that we can expect for the future:

  * It is currently possible to invite an email that you have already
    invited, it might make sense to change this behavior.

  * Resend currently sends an invite reminder instead of resending the
    original invite, this is because 'custom_body' was not stored when
    the first invite was sent.

Tweaked in various minor ways, primarily in the backend, by tabbott,
mostly for style consistency with the rest of the codebase.

Fixes: #1180.

zproject/settings.py

@@ -1018,6 +1018,7 @@ JS_SPECS = {
             'js/settings_users.js',
             'js/settings_streams.js',
             'js/settings_filters.js',
+            'js/settings_invites.js',
             'js/settings.js',
             'js/admin_sections.js',
             'js/admin.js',