Add invitation flow to the login server

[#77649316] https://www.pivotaltracker.com/story/show/77649316
Set property login.invitationsEnabled to true
Test invite users link on homepage
Fix failing integration test
Merge branch 'feature/invitations' into develop
Fix test for latest changes.

[#77649316] https://www.pivotaltracker.com/story/show/77649316

Signed-off-by: Madhura Bhave <mbhave@gopivotal.com>