replaces npm-run-all with concurrently

[ChrisBAshton]

Fixes event-stream vulnerability: dominictarr/event-stream#116

Removes dependency which has a compromised subdependency.

• Tests added for new features
• Test engineer approval

[codeclimate]

The PR diff size of 10390 lines exceeds the maximum allowed for the inline comments feature.

[codeclimate]

Code Climate has analyzed commit 71eb5e2 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (90% is the threshold).

This pull request will bring the total coverage in the repository to 97.0% (0.0% change).

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[ChrisBAshton]

Build is currently failing because concurrently doesn't exit the server when the other tasks have passed. There is a --success flag that should allow us to describe when the process should exit but I've not had much luck with it.

Have raised: https://github.com/kimmobrunfeldt/concurrently/issues/177 in the meantime.

[ChrisBAshton]

Another alternative is to stick with npm-run-all but use the NPM resolutions property to lock event-stream to v3.3.4: https://github.com/bbc/web/pull/234/files

EDIT: have raised #990. We can merge that in the meantime. Suggest we continue with this PR afterwards, as it's still worth swapping out for an unaffected library.

[ChrisBAshton]

Superseded by #990