Add streaming for backend chat

[luarss]

• streaming options supplied in body
• stream=False when doing tests.

[hillhack]

is this repo is open for gsoc? i hv gone through this issue and streaming is already implemented. But there are 3 gaps i found may be u also mean them : Gap 1: The
UserInput model has no stream field
2: The proxy in ui.py doesn't pass stream through
3: Tests use no stream field, need stream=False explicitly
#226