[META] Commons Group Privacy Revamp

Problem/Motivation

Many people have had issues with group privacy settings, especially when a group is changed from one setting to another. Currently, privacy settings are set in two places. 1) group 2) content itself. When the group is changed, it does not change past node privacy settings.

For private groups, they are still inaccessible from the group page, but individual nodes can be accessed
For moderated groups, all previously public content stays public.

On the flip side, all content that was private, stays private. Since there is no setting on individual content, I believe this might be confusing to administrators. Also, not requiring og_access changes how we interact with groups, and users have forgotten to enable og_access, causing issues after the site is built.

Proposed resolution

1) Automatically change group content privacy settings when a group's privacy settings is changed
#2034867: Reflect the content access permissions on group content nodes.
2) Add an option for admins to change content privacy irrespective of the group setting for moderated 'hidden' groups.
#2083907: Allow public post and access to content on private groups (join with approval), members can also post privately, #2058005: Have a private group, but (some) public content
3) Consider making og_access required?
#2117791: Unclear that og_access is required for group privacy settings to hide content, #2083261: Invitation-only group privacy setting does not make content private

Remaining tasks

User interface changes

Group privacy settings for content altered instead of hidden. Checkbox added to allow admins to override privacy settings on a per-content basis.

API changes

None.