Add "Validate local catalog" to footer

[jakubklimek]

See the footer on https://data.gov.cz/ - we need the same link to the catalog validator in the catalog.

I could reuse the CLIENT_CATALOG_VALIDATOR env variable, however, it contains the catalog={} param which cannot be left empty, so it would be better to change the constans/processing a bit more to achieve this. So maybe you will come up with a systematic approach ;)

[skodapetr]

@jakubklimek There is change in the configuration. I've used two properties one for landing page and one a validation. The idea was to avoid introducion of tight coupling between the components.