Improve web backend

Description

General improvements:

• Replace Gson and org.json APIs with Moshi
• Concurrent collections used for websocket sessions
• JSON content-type set to application/json
• Cache control headers set to no-store for application/json & text/plain. Max-age of 24 hours for all other file types.
• Zip download of media uses store method instead of attempting to deflate
• Site certificate end date set to intended duration
• Small formatting changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the CONTRIBUTING section of the README document.

Resolves

Mention: !2013 (merged)