Add a check for existing issues for plugin errors

[karaggeorge]

Currently we check for if an issue exists by Sentry issue ID and the label Sentry issue to avoid making duplicate issues when it comes to Kap errors. But when the issue is connected to a plugin we just link to the new issue page with the title/body filled in. So it's really easy for users to just click submit without checking for existing issues.

We should do something similar for plugins, but instead of Sentry errors or labels, we can just check the title/body with a simple GitHub api search, hosted on a lambda similar to the Kap one. The title/body should match for same errors unless the user changes it, since we clean any user specific paths with clean-stack, so I think it'll generally work. If we find a matching issue, we can just link them to that existing issue same as we do for Kap.

Trying to avoid this: