Add form validation when inviting users to Groups/Projects

Problem

At the moment, a user can invite an existing member of the group/project. This creates a lot of noise for the invitee as they receive an email invitation urging them to either accept or decline. If they accept, they're eventually shown an error that says something along the lines of "you already are a member of this group/project". All this could be prevented with basic form validation where we check if the user being invited is already a member of the group/project.

Proposed solution: add form validation that shows an alert when a user tries to invite an existing member.