[w3c/manifest] Processing the manifest is no longer a function of document URL (#834)

This change (choose one):

* [X] Breaks existing normative behavior (please add label "breaking")
* [ ] Adds new normative requirements
* [ ] Adds new normative recommendations or optional items
* [ ] Makes only editorial changes (only changes informative sections, or
  changes normative sections without changing behavior)
* [ ] Is a "chore" (metadata, formatting, fixing warnings, etc).

Implementation commitment (delete if not making normative changes):

* [ ] Safari (link to issue)
* [ ] Chrome (link to issue)
* [ ] Firefox (link to issue)
* [ ] Edge (public signal)

Commit message:

If the start URL is missing or invalid, the processed manifest simply does not have a start_url, rather than defaulting to the document URL.

If the start_url is invalid or not within scope of the document URL, the document is no longer considered installable, as opposed to the start_url defaulting to the document URL. 

Closes #668 and #669.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/834

-- Commit Summary --

  * Remove document URL from processing the manifest.
  * Update document installability rules based on document URL.
  * Be more specific about reading the document URL.

-- File Changes --

    M index.html (39)

-- Patch Links --

https://github.com/w3c/manifest/pull/834.patch
https://github.com/w3c/manifest/pull/834.diff

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