Refactor to support multiple accounts

[sirbrillig]

This refactors how the app deals with its fetching system so that it can support fetching multiple accounts at once. Those accounts can be on the same server or on different servers (supporting GitHub Enterprise Server). They can also require a proxy connection.

As a side-effect, this PR (ironically, given the name) decouples the app from the gitnews library itself and replaces it with octokit. Octokit is much better maintained and requires jumping through fewer hoops to make this app's functions work.

Warning

This is a major change to the app. While it will provide a migration from the previous version, it changes quite a lot.

Fixes #184
Fixes #185

To do:

• Create better UI for account configuration
• Create new UI for first-time user (to create a new account)
• Figure out some way to migrate old single-token configuration into account list
• When accounts change, re-fetch immediately