Add (configurable) support for custom protocols/URLs / prevent element from messing with them

**Is your suggestion related to a problem? Please describe.**

We've a document management system which generates links like e.g. `vis://08680B3E-6E1E-4C1B-B3E7-7019D5FFB910/14/5079312` which element currently automatically translates to `https://08680B3E-6E1E-4C1B-B3E7-7019D5FFB910/14/5079312` when sent via a chat message, which then won't work for us when clicking on it in a browser.

**Describe the solution you'd like**
It would be really useful if element would have support for such custom protocols/URLs. Either per se, or configurable in some way via an allow-list.