GitHub Context

**Is your feature request related to a problem? Please describe.**
I spend a lot of time in GitHub, mostly looking at issues and pull requests. The problem is the browser is too slow and cumbersome.

**Describe the solution you'd like**
Add a GitHub context to lazygit which would display issues, pull requests, and releases using the [GitHub CLI](https://cli.github.com/).

**Describe alternatives you've considered**
I can run `gh` commands in my terminal and even leverage [fzf](https://github.com/junegunn/fzf) to improve the experience.