Provide URL for raw/file/download views also for artifacts with a single file

It would be convenient to be able to download build logs without a tar container from the web interface.

You can easily view a build log inside the rendered html and a download link with a .tar.gz is also offered, but neither of them allow for consumption like curl $someurl | ....

Bits to fix:

• redesign URLs without FileInArtifact ID (!1760 (merged))
• do not force .tar.gz archives when downloading artifacts, so that the view can correctly special-case single-file artifacts (!1761 (merged))
• include links to raw/download views from the "artifact" view for build logs (!1766 (merged))