[w3c/ServiceWorker] Change the rest of the links to the new repo (#961)

@jungkees This will complement https://github.com/w3c/ServiceWorker/commit/eb7b112ac7055ff6afb4bb5a97ed72c4ac6c5c23 and updates the rest of the links (`s/slightlyoff/w3c/`).

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/ServiceWorker/pull/961

-- Commit Summary --

  * Change the rest of the links to the new repo

-- File Changes --

    M CONTRIBUTING.md (6)
    M README.md (6)
    M implementation_considerations.md (2)

-- Patch Links --

https://github.com/w3c/ServiceWorker/pull/961.patch
https://github.com/w3c/ServiceWorker/pull/961.diff

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https://github.com/w3c/ServiceWorker/pull/961