Invites to join an Org require the user to accept the invite

[beltranaceves]

Addressing #1337.

Hi, first time contributing to this repo. I wanted to validate the changes and style before writing tests.

Checklist:

1. [x] When inviting existing users, they are not automatically added to the org until they accept the invite.
2. [x] Invites for existing users cannot be accepted if the user is not logged in.
3. [x] Invites for existing users cannot be accepted if the logged in user is not the recipient.
4. [x] Invites for new users cannot be accepted if a user is logged in.
5. [x] Show pending invites on login.
6. [ ] Invites list view.
7. [ ] Cleaned up temp changes.
8. [ ] Broken test are fixed.
9. [ ] New test have been writen.

[beltranaceves]

I was tempted to add a route for logged in users that lists pending received invites, accessible from the user dropdown menu. But it seems that issues and PRs are fairly atomic, so I kept it pretty barebones