Provide short-URLs for arbitrary indico links

This could work similar to GitHub's [git.io](https://git.io/), i.e. only supporting URLs from that Indico instance. Maybe without the form to paste a URL yourself.

The shorturl could default to `https:///go/` with a random shortcut. Creating it could be done through a RESTful API like `POST /shorturls/` with a payload like `{"url": "/event/xx/whatever"}` which would either return the existing shorturl for it or create a new one.

A possible way of displaying the current shortcut and/or creating one could be an ajax-loaded tooltip on links where a shortcut makes sense.

To support a custom domain (such as `https://indi.co/`) we could have a config option that allows overriding the format for shortlinks - then the administrator of the instance would just need to configure their webserver to map that domain to actually call the `/go/...` URL.

Start by mapping the existing REST API and link-handling paths against the proposed POST /shorturls/ and /go/ endpoints. Define how existing short URLs, random shortcuts, instance-only URLs, and the configurable custom-domain format should behave. Done should include the API and redirect flow, with the optional link tooltip considered as a separate interface.

Written by the indexing model from the issue text.