Warn if a recipe requires capabilities not present on the target

[mythmon]

Recently we ran an experiment that failed because we targeted modern versions of Firefox, but we used the action preference-experiment, which is not available on modern version of Firefox. This is the kind of operations error that Normandy's capability system is designed to defend against. However, we aren't using capabilities in NDT, and so we can't leverage this information.

NDT should use the capabilities of a recipe to display warnings if they don't match the capabilities the target is expected to have. Capabilities (effectively) vary only by the version of Firefox, so pulling out the version filter object, using that to look up the capabilities, and then comparing that to the built recipe should be effective.

An example of a place to pull the list of capabilities from is here: https://archive.mozilla.org/pub/firefox/candidates/76.0-candidates/build2/linux-x86_64/en-US/firefox-76.0.normandy.json

Due to a quirk in how they are generated, capabilities are not available on Release versions, but they can be pulled from the candidates directory, which are the versions that we may potentially use for Release. The last buildN directory should be the final release version.