Add types for all API's to make express request and response type safe

[sudharsan-selvaraj]

Would be very nice to add types for the new API to avoid accidentally breaking the code in the future. At least on the responses part.

Ref: https://plainenglish.io/blog/typed-express-request-and-response-with-typescript

Originally posted by @afathonih in #944 (comment)

[afathonih]

Turned out a lot easier as we can just do it without custom typing:

async function getQueuedSessionLength(request: Request<void>, response: Response<number>) {
  response.json((await ADTDatabase.PendingSessionsModel).chain().find().count());
}