Reduce permissions requested by the browser extension

[antross]

Currently the browser extension needs to request some fairly all-encompassing permissions:

  "permissions": [
    "<all_urls>",
    "webNavigation"
  ],

This results in a prompt similar to the following stating that webhint can "read and change all your data on the websites you visit" (which some users may find scary/surprising):

As a developer tool this is somewhat expected, but I think we can do better and avoid this warning by routing all page access through the devtools extension APIs and removing the content and background scripts (still need to test and confirm this). Then the extension only gets access to the page when the devtools are open (which is all it truly needs).

[antross]

Update: testing has shown that Chromium browsers also display the "Read and change all your data on the websites you visit" text when a "devtools_page" is specified in the manifest (which webhint absolutely must have to function). Unfortunately that means we can't avoid the prompt there.

However we have a similar prompt in Firefox which lists three items:

Here we can reduce the three items to just one: "Extend developer tools to access your data in open tabs" by making the changes proposed in this issue.