(calling `Instances.set_reachable/1` only if instance had `unreachable_since`,
calling `Instances.set_unreachable/1` only if instance had nil `unreachable_since`).

Related to #85

Everything should now be configured at runtime, with the exception of
the `Pleroma.HTML` scrubbers (the scrubbers used can be
changed at runtime, but their configuration is compile-time) because
it's building a module with a macro.

Discussed in !426 (comment 7232)