For corporate deployments provide option of hiding the homeserver

### Description

In our office we now have a closed matrix server (not exposed to the internet), and everyone keeps asking "why do I have to specify the URL?". In this particular case, it would be best if Riot could infer the domain name from the current user's homeserver, or even a default specified in the config. For instance, I should be able to invite `@jeff` without having to type out `@jeff:mycompany.com`.

This should also extend to aliases: I shouldn't have to add the domain if Riot is known to be connecting to a single non-federated homeserver.

Ideally, the domain would be hidden from the user in display circumstances as well. Opening up the room settings or viewing MemberInfo would show `@jeff` and `#devs` instead of `@jeff:mycompany.com` and `#devs:mycompany.com`.

This was a major roadblock for getting Riot/matrix in our office: People just don't understand or don't want to type out the company's name every time (our company is only 5 characters).

### Version information

- **Platform**: web (in-browser)
- **Browser**: Chrome 60
- **OS**: Windows 10
- **URL**: Private. v0.12.2