Editor: Prefix non-standalone URLs with "http"

[aduth]

Fixes #1846

This pull request seeks to add an "http" prefix to links that cannot be used standalone (i.e. those with schemas, or hashbang/query/slash prefixed URLs).

Implementation notes:

The wplink plugin closely mirrors the behavior of the core equivalent plugin, which is why it is implemented the way it is. See original reference implementation:

https://github.com/WordPress/WordPress/blob/e844695e121c1a01f43f312ecd539affb004f0eb/wp-includes/js/wplink.js#L87-L95

Testing instructions:

1. Navigate to the Calypso post editor
2. Select a site, if prompted
3. Add some text to the editor content
4. Highlight the text
5. Click the Insert Link button in the editor toolbar
6. Add URL text and click Add Link
7. Note that...
   • If the URL in step 6 includes a schema, that schema is used
   • If the URL in step 6 starts with a hashbang, query parameter, or slash, it is not modified
   • Otherwise, an HTTP schema is prefixed

[lezama]

Works as described!

Not from this PR but is intentional that we don't display the hashbang on the popover?

[aduth]

Not from this PR but is intentional that we don't display the hashbang on the popover?

Yep, this is how it behaves in wp-admin too. The idea is that in the overlay display, the URL is trimmed to take up as little space is possible.