Transfer `proximity.py` from Gender Analysis Toolkit

[joshfeli]

We've already begun some work on proximity.py in PR #38 (merged into main). This issue describes the rest of the process.

We've split the conversion process into four steps, each of which work with a different part of the stack:

• Sending a Corpus id from the frontend to the backend through an API endpoint (for Document iteration) (API to backend)
• Creating a model for persisting the results of run_analysis and the other analysis functions to the database (backend to database)
• Sending the results of the analyses to frontend; building API endpoints (backend to API to frontend)
• Displaying the results of the analyses to the user (frontend to browser)
• Transferring over run_analysis function from proximity.py
• Transferring over other analysis functions, such as by_date,by_document, by_metadata, etc.