Feature Request: Ability to filter out PRs on user's forks

It's a reasonably common flow to open up a PR on your own fork before upstream to discuss with collaborators or sanity check CI that's difficult to run locally.

As is:
* These PRs are listed in the activity overview
* They contribute towards stats like closed PRs

For me, this type of activity adds noise to the insights that gh stats provide. For example: somebody may have a ton of closed PRs, which doesn't look great, but when you take a closer look it's just on their own fork which is totally fine.

To Be:
* #FixTheFilters: remove this activity from ghstats by default
* Add an option to "include activity from own forks"*

\*I don't see _much_ reason to want to include activity on forks, but perhaps somebody wants it. Once or twice when I've clicked into the PRs on forks I've noticed that people are collaborating with others before opening up on upstream, which is a good data point. But if it's easier to outright remove it, I can definitely live with that.

Start by locating the entry points that gather pull requests for the activity overview and closed-PR statistics, then inspect how a user's own forks are represented. Done means activity from those forks is excluded by default, with an option to include it when desired, without corrupting the existing statistics.

Written by the indexing model from the issue text.