Manage groups of mailing-lists

Add concept of "group" of mailing-lists.
This could be useful, if you manage many different mailing-lists in a same archive, but want to group them logically (like group "xwiki devs" and "xwiki users" under "xwiki" group).
At first, it could be used for nothing more than specific display (in the mailing-lists panel for example). It could be added as a specific Tag in pages, like mailing-lists and mail types (but maybe it could get very crowded ...). The "color" of a mailing-list could also be overriden by the "color" of the group it belongs to, if this color is specified. Maybe it could just be a specific display option at timeline level (either "use mailing-lists colors" or "use groups colors"), as it's currently the only location where the color is used.
It could also be used to specify that you want mailing-lists from a specific group, to be stored in specific Space of current wiki. But it may be a very impacting change, and I'm not sure of the possible benefits of doing that.
Had the idea of targeting a given sub-wiki, but it seems quite complex to do in practice. Would be better for this use-case, to install and configure a mail archive in the sub-wiki directly. Would also simplify, if you want to manage specific rights for these mailing-lists, to manage them at sub-wiki level.