Add links to rollbar errors for user in their admin page

[toolness]

Since fixing #2067, I thought it might also be useful to be able to link to errors related to the user that may have been reported via rollbar.

Here's an example link:

https://rollbar.com/justfixnyc-production/all/items/?query=username%3Aatul

See the Rollbar search docs for more details.

That said, I think that our front-end actually reports the person by their numeric user ID, while our back-end reports them by their alphanumeric username, which may complicate things. Also, I don't think that our error reporting from Celery includes any information about the person in question, which is a bummer.