Approval list issues

[lilyball]

Reading through the code, the approval list has several issues:

• r=me on a commit will put both the posting user and a username "me" on the approval list
• r=username on a commit is accepted even if neither the poster nor the referenced username are reviewers. This seems like a reasonably major issue. Edit: This isn't true, head_comments is actually filtered to only comments made by reviewers.
• r=metajack on a commit will put both the posting user and "metajack" on the approval list; in this instance, it's seeing the r=me prefix and not realizing that the "me" was part of a larger username
• r+ $SHA and r=username $SHA on PR comments requires $SHA to be all lowercase. Minor issue, but it really should accept uppercase as well (and the regex should require hex characters, rather than any letter, although that's moot since a non-hex char won't match the current sha anyway).

A couple of priority issues as well:

• p=100 on a commit is accepted by anyone, even a non-reviewer, even if it's on a different comment than the comment that establishes approval. Edit: This isn't true, as above, head_comments is filtered to only reviewer comments. It still allows it on a different comment but that's probably fine.
• p=100 can't be specified on a PR comment, even one that establishes approval.