Consider using bundlesize as a "check"

[aaemnnosttv]

Feature Description

We currently run bundlesize to ensure webpack-built bundles are within reasonable sizes during our JS Tests that run on Travis.

This works fine as a pass/fail but is currently implemented as a pass/fail as part of the Travis build as a whole. The executable also supports a closer integration with GitHub by integrating as a check that runs on all PRs. This could be required to pass or not for merging separately from the Travis build it runs on.

The main benefit seems to be that it makes bundle size changes introduced by a PR more visible

Example from the readme:

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Acceptance criteria

Implementation Brief

Changelog entry

[tofumatt]

This is a duplicate of #2539. In fact, #2539 is a duplicate of this, but since that one has a PR for it and discussions, let's close this and keep working on this issue in #2539.