Re-process text descriptor -> Data source representation for individual leaf criteria

[qqndrew]

Depends on #4 Allow users to re-do expansion manually

• Frontend - add button to individual textual leaf nodes and implement codeset selection GUI - assign to @hehuan2112
• Middleware - add expansion trigger API call

[qqndrew]

Middleware complete, new endpoint is

• GET /_textres/representations to obtain data source representations for a given textual span

[qqndrew]

Note that expandedCodes format has changed.

For latest example. please refer to https://github.com/OHNLP/impact-backend/blob/master/src/main/resources/sample_criterion.json