[gold] skiacorrectness.go: Extract UI routes into addUIRoutes().
[gold] skiacorrectness.go: Extract UI routes into addUIRoutes().

This refactor moves the code necessary to serve Gold's UI into a new function addUIRoutes().

This CL is analogous to https://skia-review.googlesource.com/c/buildbot/+/327326.

Bug: skia:10816
Change-Id: I14c5c46d572f9de5f70e53eed6241970c288cfa0
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/327327
Commit-Queue: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot