Shanbady/chunk size dropdown

[shanbady]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/6552

Description (What does it do?)

This PR adds a dropdown to the syllabus chat page that allows for selection of the chunk size to use when performing content file search.

Screenshots (if appropriate):

How can this be tested?

1. Checkout this branch
2. Make sure you have AI_MIT_SYLLABUS_URL="https://api.rc.learn.mit.edu/api/v0/vector_content_files_search/"
3. set the openai env var OPENAI_API_KEY (can grab from heroku rc)
4. set "botEnabled" to true in frontends/main/src/app-pages/ChatSyllabusPage/ChatSyllabusPage.tsx
5. in ai_chat/views.py set permission_classes to [] (there might be better ways of doing this but for testing locally this worked)
6. visit the syllabus chat bot endpoint http://open.odl.local:8062/chat_syllabus
7. try talking to the chat bot. change the chunk size selector to different values and see the size of chunks returned is different (it should also respond slightely differently based on this)

Additional Context

[mbertrand]

LGTM, the answers were very similar (a word or two change maybe per change in collection) but the chunks returned did differ.