Eliminate use of Github API

[Friendly0Fire]

The public Github API has a stringent limit of 60 requests per hour and no easy way for an application to connect to it securely using a token that is not associated to a specific Github account. As a result, the only way to work around this is to not use the Github API directly.

In order for this to work, the following must be done:

• [x] Devise a system to crawl and cache the Github API centrally and expose the resulting data without using the Github API.
• [x] Redesign addon manager to only use the pre-processed data for managing addons.
• [ ] Redesign addon manager to only use the pre-processed data for updating itself.

[ALERTua]

maybe you could, as an option, request the user Github API token for these requests? I wouldn't mind giving a read-only token for GW2 Addon Manager.

[Friendly0Fire]

I really don't see a need for it given that I've already reworked most of the places that used to need the GH API. Feels like it's best not to depend on it at all.