Fix large blocked users list preventing login

[tommyxchow]

Fixes #169.

Thanks to the detailed reporting in #169, we were able to discover that having a really large list of blocked users was blocking the UI and preventing the login from completing properly.

The cause

After logging in, there is an initialization process that involves fetching information regarding the logged-in user. These steps are performed sequentially through fetching with await, so the next steps must wait for the previous ones to complete.

When fetching blocked users, the logged-in user could have thousands, if not, hundreds of thousands of blocked users (one reason being bots). This will cause "jank" and essentially freeze the UI until all users have been fetched.

Furthermore, the Twitch API only allows fetching up to 100 blocked users per request, with a limit of 800 requests per minute. Therefore, when too many blocked users are fetched, it will prevent the login initialization from completing.

The fix

In order to fix this, I've removed await from the request and have opted for a normal async future with a completion handler. This will prevent the main thread from being blocked and perform the request in the "background".

To account for the 100 blocked users per request and the Twitch API limit, I've added a delay of 150 milliseconds between each request. This means that total requests for blocked users are capped at 400 per minute, half of the 800 per minute cap previously mentioned. Theoretically, this will prevent requests from throwing a rate limit error.