swarming: Fix CSP for display_server_url.
swarming: Fix CSP for display_server_url.

It should end in '/', otherwise browsers expect exact URL match (not prefix
match).

R=maruel@chromium.org, kjlubick@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2621423003

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py