:raised_hands: Add support for multiple sidekiqs

This is so we can have iHat and tahi running on the same machine without one sidekiq fighting over the other.

Make sure to have `REDIS_PROVIDER` set in your .env.
How does this work on Heroku? `ENV['REDIS_PROVIDER']` evaluates to the string `REDISCLOUD_URL` on staging. So the above ends up evaluating to:

```ruby
Sidekiq.configure_server do |config|
  config.redis = {
    url: "REDISCLOUD_URL",
    namespace: "tahi_staging"
  }
end

Sidekiq.configure_client do |config|
  config.redis = {
    url: "REDISCLOUD_URL",
    namespace: "tahi_staging"
  }
end
```

Is the `url` key required? Is this initializer evaluated too early to try `config.redis.merge namespace: "tahi_#{Rails.env}"`?