Modularize manifest files

[emilgoldsmith]

This resolves #84.

Note: I have a temporary final commit that I will delete and force push before merging, it contains the built manifests with their old names in order to be able to review what the scripts output will look like in this PR while reviewing changes.

My biggest thoughts are actually on the actual manifests more than the code. Like for example:

• there are small differences along the 4 original files in content_security_policy, could they all be the same?
• I saw weird things in devFirefox where it was requiring permission for management and only included one image size instead of the normal 3, while the production firefox manifest didn't have any of this (it didn't require management and had all 3 images). I'm assuming this is an oversight exactly because manifests weren't modularized?
• In the dev manifests they always include img/ and fonts/ in web_accessible resources is this really needed? Is this an outdated thing as well? Don't we just fetch them normally from the extension folder while in dev as well? And if we don't shouldn't we just as well?

Yeah so the above questions would be awesome to have some dialogue on. The one I'm most confused about is the content_security_policy. I assume there shouldn't really be a difference between Firefox and Chrome there right? So shouldn't we try and come up with a common CSP that we can apply to all of them? If we can't I have added support in the script to merge CSPs though, so if needed we can also do that, but currently I know that what I have done is not correct anyway.

[emilgoldsmith]

Yep, thanks for the review! I'll look into your comments and try and heuristic my way through finding a balance / correct manifest format.