[WIP] Search api

[franckc]

Checklist:

• [ ] Code contains relevant tests for the problem you are solving
• [ ] Code has been formatted with autopep8 --in-place --recursive --a --a .
• [ ] Ensure all new and existing tests pass
• [ ] Update any relevant READMEs and docs
• [ ] Submit to the develop branch instead of master

Description:

This PR implements a new /api/search/listings API that can be used by the DAPP to search listing. It provides a couple simple filters (by category and by location) - additional filters can be added later on after we flesh out the requirements for the DAPP search capabilities.

Note: this is Work In Progress, not ready to be pulled yet:

• Unit test coverage needs to be improved
• This PR has a dependency on a couple of other PRs (https://github.com/OriginProtocol/bridge-server/pull/65 and https://github.com/OriginProtocol/bridge-server/pull/64) that should get pulled in the develop branch first

[franckc]

Closing this PR - implemented as part of #81