R1:a8d6af0f42a8

Infrastructure changes to support scoped symbols

Summary:
- import_project_symbols supports an optional extra field, which is the
context of the symbol.
- Symbol query can take a symbol argument, either as a parameter or a
URL component (so you can now jump nav to `s Zerg/rush`, for
example).
- Conduit method not yet updated. Will do that later.

NOTE: Not providing a context is distinct from providing an empty
context, because an empty context stands for top-level context, i.e.
functions and…