Wire up ResourceMapper

[RubenVerborgh]

The ResourceMapper introduced in #643 should be wired up, such that the new URL/filename correspondence is used.

The steps are:

1. wire up the LegacyResourceMapper first (Wire up LegacyResourceMapper #661)
2. replace LegacyResourceMapper by ResourceMapper and adjust tests (Wire up ResourceMapper #662)

[RubenVerborgh]

As remarked by @timbl, people might still want to keep using the LegacyResourceMapper, so we could add a configuration option for that.