Project: allow maintainers to remove themselves if other maintainers exist

[agjohnson]

We currently block maintainers from removing themselves in the maintainer list. This is partially to disallow orphaning a project. In the case of a project with multiple maintainers, it should be possible to self remove from a project. I don't recall if we allow this at the form side, but it seems like something we should support.

[stsewd]

The app does allow removing yourself and checks that you are not the last owner.

https://github.com/readthedocs/readthedocs.org/blob/a405d9df02fd6a60a8d78b4d040d606973e704a0/readthedocs/projects/views/private.py#L552-L570

So, this is just the template that needs updating.

We even expose a context variable

https://github.com/readthedocs/readthedocs.org/blob/a405d9df02fd6a60a8d78b4d040d606973e704a0/readthedocs/projects/views/private.py#L539-L539