Add UI for editing multi-preference-experiment recipes

[rehandalal]

Not the prettiest thing in the world but it gets the job done.

There's a little bit of a gotcha that I couldn't solve without a massive overhaul of the forms (which didn't seem worth it) --- if you are typing a preference name and it happens to match an existing preference name, the existing pref will get overwritten with the one you are currently editing. In most cases I'm assuming users will be copying and pasting these pref names in so this should not be a problem.

[bors[bot]]

Merge conflict

[mythmon]

I'm not sure how to cleanly fix the conflicts in the stylesheet.

[rehandalal]

bors r+

[bors[bot]]

Build succeeded

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