[error-monitoring] Add process to record duplicate error

Often, a single error will surface in 3~4 different variants depending on browsers and module/nomodule splits. They should only get tracked in one GitHub issue, and that issue should be linked to each of the associated errors in Pantheon. First, we need to provide instructions for this in the page. Then, it would be useful to provide a way to link an existing issue from the UI. Finally, it would be helpful to provide a list of recent Error Report issues to make it easier to find recent repeats.

Steps:
- [x] add instructions for dealing with dupes
- [x] add backend endpoint to link issue to dupe
- [x] add form field for each issue to link an associated issue
- [ ] comment on GitHub issue with link to additional error group
- [ ] add list/dropdown of existing recent error report issues