Add feature to invite users to host team using URL

Very similar to #1346.

## Deliverables

Add a feature to invite new team member to a host team using a unique URL. Also, don't forget to write unit tests. :)

## Steps

1. **user1** enters the email of his teammate (let's say **user2**)
2. Send an email to **user2** (if he/she is registered on EvalAI) with a unique URL
3. **user2** clicks on the URL to accept the invitation to the team.
4. *user1* gets an email saying that "**user2** has accepted your invite to team " (we can iterate over the email message later)

### Some important points to note

- [ ] Checks performed before inviting to the team should be similar to current invitation logic