auth: manage organization access

[bradzacher]

I didn't realise for a while that the reason I didn't get any notifications from my organisation repos is because I hadn't requested access for gitify.

I only realised this was a problem because it's listed on your FAQ on your Github

it'd be good if it did something like the following:

• During the sign up process, look for any organisation repos and ask the user to request access.
• Keep a record of what organisations the user has declined to get access to.
• When occasionally check to see if there are any new organisations the user has access to, and provide a notification prompting the user to request access.

Failing that, just display a message during signup telling the user how to request organisation access.