Allow external highlights

[Roznoshchik]

All highlights are currently connected to an article inside of the app. But this is a limitation that shouldn't be there as you should be able to import highlights from anywhere to start using with the platform.

The easiest might be to add a boolean for external=True and an externalSource=... fields to the highlight model.

Then the highlight page should have a create highlight button to allow for manual creation.
It should also have an import highlights button that uses something like this import code as base.

Also the web extensions can then be updated to allow for sending highlights and not just for sending the url to the app.

[Roznoshchik]

This is pretty much done. Highlights have been given a source field that can be used for storing the source of a highlight if there isn't an article attached.