luci-base: allow themes to provide sysauth.htm

Support for sysauth_template was (inadvertently) dropped in refactorings
to support the json menu construction.  This does not restore that
functionality, which allowed different templates for every node in the
dispatcher tree, but provides an alternative mechanism that allows a
theme to provide a sysauth.htm template file instead.

Tested-by: Karl Palsson <karlp@etactica.com>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>