Support a different default domain behavior

[acelaya]

Summary

Shlink currently has a not super predictable behavior regarding the default domain.

This is because it used to support only one domain, and when multi-domain support was added, it had to maintain backwards compatibility.

However, this behavior could be changed for users coming to Shlink now, via a config option that tells how it should behave.

See this discussion for details.

[acelaya]

There hasn't been a lot of attention for this, which feels like it's a niche use case, and most of the people are happy with how Shlink works.

Closing.