Handling of nested browsing contexts.

[markafoltz]

This PR updates the spec with the resolutions to Issue #79 (Presentations from within nested browsing contexts). It:

• Patches the HTML5 spec to add an allow-presentation token to <iframe sandbox=>
• Provides UI guidance that the UA should tell the user what origin is requesting presentation.

It also removes links to a couple of resolved issues.

[anssiko]

Thanks for the pull! I committed some further clarifications to this branch:

• Defined "presentation is disabled" we can reference elsewhere in the spec
• Hooked that "presentation is disabled" into the entrypoint navigator.presentation (assuming the intended behaviour is to return null if presentation is disabled)
• Made the informative text in the Sandboxing section a note for clarity

@mfoltzgoogle Feel free to merge this PR, if you're happy with the clarifications.

[markafoltz]

Thanks for the edits @anssiko. Merging and closing issue.