Allow users to invite team members by email

Problem to solve

Back in https://github.com/gitpod-io/gitpod/pull/4490, we've added the ability to invite team members by sharing a unique invite URL. See relevant specs in https://github.com/gitpod-io/gitpod/issues/4421.

The initial specs included a way to invite team members by email which we decided to drop to limit the implementation scope in the spirit of MVC (minimum viable chage).

From a data perspective, we've been noticing fewer that expected teams with more than 1 or 2 team members which could be a sign of friction while setting up a team. ​See relevant discussion (internal).

Proposal

We could consider adding invitation by email to make it easier for users to invite others instead of sending an invite URL. The initial specs for inviting team members in https://github.com/gitpod-io/gitpod/issues/4421 already included an early design draft, see screenshot below.

Step 1: A good first MVC could be to just send the invite link to the typed email addresses. See relevant discussion (internal). Cc @jankeromnes

Step 2: In the long run, a safer approach could be to a) keep track of pending invites so that you can revoke them anytime and b) send a unique invite URL to each user added via email.

The issue names no implementation files or tests. Start by reviewing PR #4490 and issue #4421, then compare the proposed Step 1 and Step 2 scope; completion criteria are not defined beyond sending invite links to typed email addresses.