Isolate URL/filesystem correspondence

[RubenVerborgh]

Currently, the knowledge on how URLs correspond to the filesystem is spread out across different handlers. We need to a) gather it in one place b) change that place to follow the Design Issue.

This requires the following steps:

• Writing a URL/file mapper (done in Add ResourceMapper class for URL/file mapping #643)
• Refactoring all handlers to use the mapper (Wire up LegacyResourceMapper #661 and Wire up ResourceMapper #662)

[RubenVerborgh]

Following up in #661 and #662.