Allow all contributors to `bors try` their own PRs

First of all, I'd like to thank the contributors to bors and the people running the free bors service -- we really appreciate it!

I'm posting here because I'm running into an issue with allowing all contributors to an open project I'm apart of to be able to bors try their own PRs. Our CI is split between a couple of different services and we don't want to run the full test suite on every commit. We do, however, want to allow users to run the full test suite on their own as needed. I wasn't able to find information about how this could be done with bors. Currently, users are getting a "permission denied" error when attempting to bors try their own PRs.

The "synchronize" section in the "settings" page on the dashboard looks like it could possibly solve this issue (does synchronize=pull add everyone with pull permissions (everyone on GitHub?) to the member list?). However, I don't know what it does and couldn't find an explanation. Something like a tool-tip or dynamic bit of text, when an option is selected, explaining what the drop-down options do would be really helpful!

Ideally for us, the permissions to try and retry would only apply to pull requests opened by that user, but allowing all users to bors try on any PR would probably be fine in the short term for us too!

Let me know if I can help with this! Either by updating the docs / adding a tool-tip to explain how this is possible in the current system, or by contributing to a new feature if this is not currently possible and something you all are interested in supporting!