FIX: Invalidate database theme cache when hostname changes (PR #9908)

Hostname can vary per-site on a multisite cluster, so this change requires converting the compiler_version from a constant into a class method which is evaluated at runtime. The value is stored in the theme DistributedCache, so performance impact should be negligible.

GitHub

This seems good but we should bear in mind that every theme will be recompiled when we deploy this.

We can change this to SiteSetting.force_hostname = "someotherhostname.com"