GFM spec redesign

Should now be much clearer about what each spec is actually testing.
For example, instead of testing stuff like link classes and titles in
every single call, we only test those things once, in their own specs.

Instead of doing this:

    link_to(commit.id, project_commit_path(project, id: commit.id))
    Note.create(noteable_id: commit.id, noteable_type: "Commit", ...)

It lets us do this:

    link_to(commit.id, project_commit_path(project, commit))
    Note.create(noteable: commit, ...)

Issue status changed notifications

nginx vhost standard

Fix milestone API specs

Increased model spec coverage

Closes #1331