Summary:
This makes two small tweaks based on a user's questions about sharing
sites publicly for a self-managed installation:
* The support user `support@getgrist.com` is made configurable with
`GRIST_SUPPORT_EMAIL`. This came up because only the support user
can share material with the special "everyone" user. This restriction
was added to avoid spam.
* Regardless of public sharing settings, for our SaaS we had
decided not to list public sites to anonymous users. That is
somewhat a question of taste, so a `GRIST_LIST_PUBLIC_SITES` flag
is added to override this choice.
Public sharing isn't in a well polished state, and this diff doesn't
advance that, in fact it adds a new wrinkle :-/
Test Plan: existing tests pass; manual testing
Reviewers: jarek
Reviewed By: jarek
Subscribers: jarek
Differential Revision: https://phab.getgrist.com/D3663
ab3cdb62ac