Select blocking webpage from browser history

[hayribakici]

Fixes #87.

This PR allows to users to add a page to the blocking list from the browser history. Since accessing the user's history needs the history permission, it adds this permission as optional_permission and users can opt-out any time in the browsers extension settings.

The default behaviour is the regular interface, where users can add the url manually. However, there is a possibility to allow impulse-blocker to access the browser history (see image below) as the browser asks the user, if impulse-blocker can access the history.

Once the permission is granted, the input value is changed to a dropdown item.

As addressed by @raicem the urls appear multiple times. I couldn't get that fixed, so I decided to only show one item when typing in the dropdownbox. This can be changed, though.

And finally, once the permissions are revoked (e.g. the browser addon settings), the interface falls back into default.