[feature] Hide groups from the homepage

More and more, I use groups to apply filtering rules. (To avoid repeating the same filter or notification settings multiple times)
However, these are 'technical' groups, and I find they shouldn't be displayed on the homepage.
(In the same time, I have groups with nothing in them (no filters, no notifications), just to group links.)
Adding a small option on group setting to hide groups could be nice.
To satisfy the more demanding users, we could imagine a 'show all groups' link for those who might want to view them temporarily.

Start by locating the homepage group rendering and the group settings entry point, then trace how group preferences are stored and applied. Done means a group can be marked hidden, hidden groups are omitted from the homepage by default, and users can temporarily show all groups.

Written by the indexing model from the issue text.