The lightweight beaconchain explorer was renamed to `dora` (Dora the
Explorer).
This PR renames all explorer related variables & references accordingly.

I've also changed the explorer db path to a local file instead of the
`:memory:` placeholder.
The in-memory database caused issues as the content is being wiped when
all db connections are being closed.
I've just used `/dora-database.sqlite` as path. Maybe some of you can
give feedback if that's the right location for it :)

Replacement for #219, as the renaming has been reworked in a new branch