Simple file validation and HTML5 zip correction

[rtibbles]

Summary

• Adds simple validation of image, video, and audio files
• Adds utility for creating predictable zip files that gives an exact MD5 match for those created by ricecooker
• Adds validation for HTML5 files
• Adds cleaning to process HTML5 files to remove unnecessary folder nesting
• If index.html is not present, finds first available html file to use as entry point
• Rezips all uploaded zip files as predictable zips to have parallel implementation to ricecooker

References

Fixes #4013

Reviewer guidance

I think the 'first available html file' might be better handled by asking the user which one they want to use, but this would require a lot more work to enable.

Testing this is best done by taking an existing HTML5 app and messing about with it, unzip it, put its contents in a bunch of nested folders, and then rezip it. If appropriate, rename the index.html to something else and make sure it still works (this will only work reliably if there are no other html files in the root of the zip).

[akolson]

Changes LGTM and manual QA checks out -- Test cases, covering all edge cases, gave extra detail on what the code was doing. Thanks @rtibbles!