web: Add further APIs

[DavidKorczynski]

We recently added logic for extracting more data from the core program analysis parts, and we should expose this in APIs that provides this data in a unit-like form, e.g.

/api/cross-references?project=XX&func_key=yy
/api/function_source_code?project=XX&func_key=yy
/api/function_signature?project=XX&func_key=yy
/api/type_data?project=XX&typename=YY