What? Why?

Closes #325

This change allows a user group to be made public, so all its associated datagroups are visible for all LX users. To configure this in the DB, add a row to user_group_memberships with the user_group_id we want to make public and user_id = -1 (i.e. the "public user").

What should we test?

• Create/choose a user group (with id X) that has at least 1 associated data group
• Log in to LX as a user who is in the user group. Confirm they can see the data layers.
• Log in to LX on a separate browser as a user who isn't in the user group. Confirm they can't see the data layers.
• Add a row to user_group_memberships with the user_group_id = X, user_id = -1, and access = 1 (readonly).
• Refresh the app and confirm both users can now see the data layers. The first user should still be able to edit the datagroup (e.g. edit object descriptions or copy a map object to the datagroup), but the second user shouldn't be able to edit.
• Delete the user_group_membership you just added and confirm the state has returned to what it was previously.

Release notes

Deployment notes

Documentation updates