hgweb: move archive related attributes to requestcontext
As part of this, "archive_specs" was renamed to "archivespecs" to align
with naming conventions.
"archive_specs" didn't technically need to be moved from hgweb. But it
seemed to make sense to have all the archive code in the same class.
As part of this, hgweb.configlist is no longer used, so it was deleted.