Desktop does not accurately reflect the latest pull requests from GitHub

[billygriffin]

Please describe the problem you think should be solved

Problem 1: The list of pull requests in GitHub Desktop does not quickly reflect the latest information about what pull requests exist from github.com. In broader terms, someone has proposed changes to the code base and I don't have any knowledge of it because GitHub Desktop doesn't reflect it until later or until I manually click fetch with the pull requests tab open.

Problem 2: When it does fetch the latest pull requests, the list is incredibly slow to load and therefore its utility is limited. This was brought up in #5448. In broader terms, I want to be able to see what work has been proposed to merge into the code base, and the list of things takes forever to load.

Example of problem 1:

Before fetch:

After fetch:

As you can see, I wouldn't have known the newest pull request (#6981) existed until I explicitly clicked "Fetch" with the pull requests tab open and then waited what felt like a long time for the list to load in (it was around 10 seconds total to load 41 PRs).

Example of problem 2:

cc: @niik

[billygriffin]

@niik An interesting point was brought up by @tierninho in #3628, where he pointed out that if you rename a branch that has a PR in Desktop, you can get into some interesting situations. I'm not sure that's a thing that happens enough to make it worth solving, but wanted to at least make that decision consciously as part of this effort.