1. this was overriding the real defaults
2. it's documented in `config/config.md`

And correct pleromaFE config.

These are all names that are used for domain.com/:route routes or projected to be.

Also rename seconds_retained to seconds_valid since that's how it is
now. Put it down from 180 to 20 seconds. The answer data is now
stored in an encrypted text transfered to the client and back, so no
ETS is needed

This allows to set-up an arbitrary directory which overrides most of the
static files: index.html static/ emoji/ packs/ sounds/ images/ instance/
favicon.png.

If the files are not present in the directory, the bundled ones in
priv/static will be used.

Closes: !551