Fix sourcemaps in Sentry

Description

Our Sentry clientside is not able to show JS sourcemaps, errors appear when displaying events:

Example event: https://new-sentry.gitlab.net/organizations/gitlab/issues/153532/?project=4&query=is%3Aunresolved&referrer=issue-stream#tags

Possible solutions

Sentry provides two main ways to get sourcemaps in the issues, by uploading them or by fetching them directly:

• https://docs.sentry.io/platforms/javascript/sourcemaps/uploading/webpack/
• https://docs.sentry.io/platforms/javascript/sourcemaps/uploading/hosting-publicly/