Re-work dataset invite links

[dignissimus]

From what I remember, dataset invite links are long and take up too much space. They should be shortened.

[dignissimus]

If I recall correctly, I am using bcrypt. This is overkill. It makes sense to switch to invite IDs and store invite links in a database which allows invite links to be revokable.

One decision is whether user access is tied to an invite link or if an invite link only grants access to a user.

For me, the latter makes most sense and might be the best maintainability wise thinking about both code design and implementation.