Separate reachability and check source info in detail views

[lippserd]

Quote from #2735/dnsmichi:

check_source and is_reachable are not necessarily bound to each other.

is_reachable is determined by host/service dependencies in Icinga 2 and updated accordingly, if i.e. a parent fails. If there are no dependencies defined, it will always be "reachable".

check_source is just the node name where the plugin was executed.

I'd suggest to move Reachability into a separate line, away from the Check Source. This removes possible questions.