Return margin calls in `get_order_book` API

**User Story**
It seems that `get_order_book` API was designed for retrieving a "combined" order book. It makes sense to add margin calls into the result if price of margin calls is in the range, because traders can place limit orders to buy into the margin calls.

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.
- [x] API (the application programming interface)
- [ ] Build (the build process or something prior to compiled code)
- [ ] CLI (the command line wallet)
- [ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
- [X] DEX (the Decentralized EXchange, market engine, etc.)
- [ ] P2P (the peer-to-peer network for transaction/block propagation)
- [ ] Performance (system or user efficiency, etc.)
- [ ] Protocol (the blockchain logic, consensus, validation, etc.)
- [ ] Security (the security of system or user data, etc.)
- [X] UX (the User Experience)
- [ ] Other (please add below)

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Feature Request
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start by locating the get_order_book API entry point and the existing margin-call handling; the issue names no files or tests. Determine how in-range margin calls should appear in the combined order book, then add coverage showing the expected result and verify the API behavior.

Written by the indexing model from the issue text.