Rename GQL_URN to DEEPLINKS_GQL_PATH, and GQL_SSL to DEEPLINKS_GQL_SSL across all repositories

[Konard]

URN is for books and standards. To use for for GQL path/url is just wrong.

https://stackoverflow.com/questions/57446486/name-of-the-portion-of-a-url-that-follows-the-scheme
https://stackoverflow.com/a/38204721/710069
https://www.rfc-editor.org/rfc/rfc3986#section-4.2

The closest idea is relative reference of URL, but it contains // which we don't use in GQL_URN env.

Other variations:

UriWithoutSchema
UrlWithoutSchema
AuthorityWithPath

SCHEMELESS_URL

And may be we should use even better way to do it: deep-foundation/hasura#15 (this way may be optional)

[Konard]

@ivansglazunov do you have arguments against breaking change here?