Sample sentry errors on frontend

Created by: jhchabran

There are no reasons to have hundreds of occurrences of the same error being reported. By sampling them, we could avoid spending our quota too quickly while retaining the same signal.

We probably would want this to be configurable, but before writing all that plumbing, I though we'd want to see how it affects the reports first.

Test plan

Should pass CI, and the change itself is merely a parameter on the sentry client.