Invite by url

This was requested back in 2014 [invite by url #603](https://github.com/keybase/keybase-issues/issues/603) and the reason for not implementing was:

> We are throttling invitations by email address on purpose, so we're unlikely to give out general invitation links

Now that keybase is more mature and growing rapidly with the ability to add people via email and other platform's user names, I would like to renew this request for invite links.

Providing Invite likes fills a space that sits between public groups and secret groups. Just as you so clearly explained in the keybase blog post dated November 20, 2017 "New Teams Features" where you announced that "You can now put teams on your profile" and explained your new understanding of _why_ someone would want to do this, the case for _invite links_ or _invite by url_ is one of facilitating the growth of community.

The ability to hand out invite links or post them publicly would allow people who don't remember the name of your team, or even the name "keybase" to easily be presented with the [download keybase](https://keybase.io/download) page if it's not installed or to seamlessly request to join the team in questions simply by following the link.

**In essence an invite link would simply automate the process of requesting to join a particular team.**

This would greatly facilitate the adoption of keybase and the ability to grow new communities without compromising the security of a private team, since the team would remain private and individuals would not be signed into the team until an administrator approved them.

Start by reading the linked prior request, invite by url #603, alongside this issue's description of invite links and team approval. Done means a link can direct people to Keybase or let them request access to a particular private team without joining before administrator approval.

Written by the indexing model from the issue text.