Updates to the web app manifest and association file URL handling API. (#12)

* Some major updates to the explainer.

* Fix naming.

* Minor edits.

* Rename handle_urls to app_links.

* FIx markdown and naming.

* Edit iOS notes.

* Edit windows and android notes .

* Edit privacy consideration.

* Edit security considerations.

* Edit user preferences.

* Edit browser changes.

* Edits to validation and revalidation. More abstract because web-app-origin-association is a recommendation, not a requirement.

* Edit association file examples.

* Edit wildcard matching.

* Edit manifest examples.

* Edit proposed changes.

* Reword goals, non-goals, and proposed solution.

* Add relation to other proposals section.

* Attribute input from Matt.

* Fix mailto link.

* Minor language edits.

Author: LuHuangMSFT