Add vcs revision to Sentry events

We have found that the build number is not a useful metric to cut the Sentry data by, as the JavaScript files are related to Common and not specific applications. Therefore, we have chosen to use the vcs revision number from the CI build instead which will group changes back to the Git SHA instead.

This PR also scrubs the md5 hash from the culprit JS file before sending it back to Sentry to avoid the same error being reported as a new unique by different builds.

/cc @rich-nguyen @ironsidevsquincy @mattosborn 
Revision numbers are quite long, why not just grab the first few (say 6) characters and report those.

Github does something similar which means this ends up in the right place https://github.com/guardian/frontend/commit/a26fd4

`revision.take(6)`