Add new supported visit type 'maven'

Without this, the following query fails in the archive [1].

While without specifying anything [2], it works.

The following commit should fix the issue with the [1] link.

[1] https://webapp.staging.swh.network/browse/search/?q=repo1.maven.org&with_visit=true&with_content=true&visit_type=maven

[2] https://webapp.staging.swh.network/browse/search/?q=repo1.maven.org&with_visit=true&with_content=true