Update my groups page headings for a11y navigation

[piet0024]

During a11y testing the user suggested we update our page heading structure to allow for easier navigation. Look to add a visually hidden h2 for "my groups" just before the groups list. Put the group list links in h3.

[LecCory]

Group link wrapped in h3, hidden h2 now available when the active link is My Groups. Uses the groups:yours key from the mod/groups/language files to toggle between both languages