feat(ui): Display invite links on the staff page

[mtiller]

This adds the invitation URL to the server response for invitations and it then leverages that URL on the client. The hope here is to address issue #247 and ideally just make it easier to use Meli without having to have a working email server to send out invites.

Design note: I very deliberately pass a URL back from the server instead of just the bare token. The reason is that if I pass back a bare token then the UI will have to formulate the URL which means duplicating that logic both client and server side which I have a visceral dislike for (very bad code smell, IMHO). This way, only the server needs to know how to construct the URLs and it can unilaterally change the scheme if it wants with zero impact on the client. Hypermedia for the win.

[gempain]

@mtiller thanks so much for the contribution ! Will release this tonight. I think we should update the docs as well to emphasize this. It seems that the lint stage of the pipeline isn't passing, could you take a look ? Running the command eslint --fix should fix things.

[gempain]

@mtiller I tried to modify your branch to save you time but it seems I can't. Happy to release a new beta with all your PRs once this one is merged 😄