Make PostFetcher and UserFetcher raise better exceptions.
Move the upserting of posts from JSON to PostUpserter.

Also, simplify JSON processing by not blindly importing `domain` and `slug`; instead, these will be in inferred from the provided post URL. We're now just checking this URL to have the same host as the source URL used to fetch the JSON, trusting individual nodes not to much up their own data. :-P
Update '.gitignore'
Change 'config/deploy/production.rb'
Remove appsignal stuff
Don't use S3 datastore
Deploy from fork repo
Set correct Ruby version
Don't use chruby
Use 'capistrano/rbenv'
Fix dotenv config

We don't use 'dotenv-deployment' for production because it doesn't work
for our setup.
Remove default tagline
Remove turbolinks crap
Fix `deploy:restart` capistrano task
Don't display "Login" link