add feature flag to enable setting a group as "global"

[ChristopherJohnston]

For self-hosted instances - particularly those hosted on a private server - sometimes we want a group to be visible to all users regardless of whether or not they've been given the link or have the group in their recent list.

This PR adds a GLOBAL_GROUPS_ENABLE feature flag which, when true, allows for a group to be set as global in its settings page and displays a list of global groups in the groups page.

Groups Page

Group Form

This is a re-submission of #162

[ChristopherJohnston]

@scastiel thoughts on this? I think it is a useful feature for self-hosters

[ChristopherJohnston]

@scastiel bump

[ChristopherJohnston]

@scastiel -> solved merge conflicts from 1.14.0 update