directory listing API

[vladak]

This change introduces directory listing API.

There are couple of limitations:

• only directories are supported at the moment, i.e. one cannot get the properties of individual file
• not possible to list source root directly
• There is no paging (like for the /history API endpoint); it is assumed that the number of entries in a directory is fairly low (hundreds of files).
• the tests ought to have bigger coverage (DTO entry comparison, with/without extras, with path descriptions, authorization test, etc.)

At one point I contemplated moving the HTML production to a JSP, like it is mentioned in the comment in DirectoryListing#extraListTo(), however decided not to because the tests rely on the HTML/XML parsing.