Enable users to remove themselves on the `/groups/{pubid}/edit/members` page

See also: https://github.com/hypothesis/h/issues/9136

Currently users can't use the _Members_ table on the `/groups/{pubid}/edit/members` to remove themselves from the group: there _is_ a row in the table for the user's own membership, but (unlike other rows in the table) there's no button for removing this membership.

Add a button so that users can use the table to remove themselves from the group.

Notes:

* This should be a frontend-only change: the backend API for removing members already allows a user to edit their own role.
* Unlike removing someone else, removing yourself from a group **cannot be undone**: once you've removed yourself you may not be able to re-add yourself (unless the group is an "anyone with the link can join" group and you have the link). We need to show a warning dialog to this effect, importantly including the fact that this can't be undone.
* Removing yourself from a group means you no longer have access to the group's `/groups/{pubid}/edit/members`, which is the page that you're currently on. So the frontend will have to redirect to some other page.
* This would interact with bulk editing (https://github.com/hypothesis/h/issues/9145, not yet implemented at the time of writing): if the user can select multiple rows and remove all of those members at once, what if one of the selected rows is the user's own row? It was suggested that we solve this by making the user's own row not-selectable but that could be confusing: if the user has clicked the "select all" button but it actually hasn't selected one of the rows.