Change Committer's Agreement to a web page that must be acknowledged

https://bugzilla.mozilla.org/show_bug.cgi?id=1112600

We also need to delete
https://static.mozilla.com/foundation/documents/commit-access/committers-agreement.pdf
(and any other parallel .odt or other forms of the doc) as no longer being necessary, but I can't find that file in the bedrock repo, so it must live elsewhere.