Rename repo to sourcegraph-sentry

[sqs]

• For consistency with other extensions: https://sourcegraph.com/search?q=repo%3A%2Fsourcegraph%2Fsourcegraph-
• Because if people in other GitHub orgs make Sourcegraph extensions (or fork ours), they would want to follow a naming pattern of sourcegraph-whatever not just whatever, so we should pick one consistent way for the whole community

The extension itself should still be named sourcegraph/sentry not sourcegraph/sourcegraph-sentry.

[vanesa]

Ok, done.