[w3c/manifest] Privacy (#441)

Address some of the privacy/security issues that came in as part of PING. 

@npdoty, would appreciate your review. 
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/441

-- Commit Summary --

  * Feat (start_url): add privacy consideration (closes #399)
  * Feat (installation): add sec consideration about timing attack (closes #400)
  * Feat (installation): users should vouch for sensitive information (closes #401)

-- File Changes --

    M index.html (45)

-- Patch Links --

https://github.com/w3c/manifest/pull/441.patch
https://github.com/w3c/manifest/pull/441.diff

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