[WIP] Split the build targets to span web, chrome extension, and firefox extensions

[prashnts]

The base plan is to split the webpack entrypoints, and build directories. This should not change the prod/dev differences since the target parts would be included only in base webpack configuration.

More details to follow.

[prashnts]

Major Change List:

• Removed <allhosts> permission.
• Refactor to support activeTab api.
• Optimize base content script, to minimize load time of the popover.
• Pack dotatlas vendor builds instead of loading it from aws.
• Split modules from mixins/utils to mixins/react-helpers and mixins/dom-helpers.
• Specify target browserlist to reduce build size and eliminate polyfills for natively supported objects.
• Stub browser proxy object as a contextual alias of chrome and browser runtime extension apis.

I'm splitting this PR further to implement the targeted builds.