Address duplicate team member bug. Fixes #1742.

Fixes duplicate recruits test and adds scoped uniqueness validation to
TeamMembership. Also adds guard to Team#recruit to remove existing
members from invitees. Performs this same subtraction when sending out
notifications in routes/teams.
Merge pull request #1846 from tbloncar/tb-team-twice

Address duplicate team member bug. Fixes #1742.