Self-hosting improvements

Most of these are "nice to haves" back-ported from Zerda.

Revamp how we load configuration. Config is no longer hard-coded in knexfile.js.
- Can now specify config with --config instead of as a positional argument.
- npm start path/to/config.json becomes npm start --config path/to/config.json
- npm run knex migrate:latest path/to/config.json becomes npm run knex migrate:latest -- -- --config path/to/config.json
  - Yes this sucks. We need to escape both npm and knex arg parsing.
- npm run register path/to/config.json becomes npm run register --config path/to/config.json
Make registering commands a little simpler. Now you just do npm run register -- --config path/to/config.json.
Modified hosting.md to have clearer self-hosting instructions.

Other stuff

(bugfix) Cleaned up an old JS import I accidentially left in one of the database migrations during the port to TypeScript. Now migrations should run properly on a fresh install.
Add engines information to package.json.
Add information storage disclosure to README.
Improve error reporting for interaction errors that fall through.
Proactively print informative error when the bot is started without a valid database.
Security update for dependencies (npm audit fix).