Add a widget for multi-pref-experiment branches

[mythmon]

It's not the best UI, but it fits in the space and all the data is available, which is more than can be said for the previous version. Without this change there is simply no way to see the preference changing or the value it is changing to.

Before

After

Fixes #1078

[bors[bot]]

:v: mythmon can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

[mythmon]

This might fail due to security updates, but I'll give it a shot.

bors r= @rehandalal

[bors[bot]]

Build failed

• ci/circleci: lint

[rehandalal]

bors r+

[bors[bot]]

Build succeeded

• ci/circleci: build
• ci/circleci: docs
• ci/circleci: test
• ci/circleci: lint