[CLOUD-18] event_log: validate URL upon responding to client
Created by: unknwon
Instead of going with the proposed solution in https://sourcegraph.atlassian.net/browse/VUL-6 (which involves a little bit hack), I found a simpler way to sanitize/validate the event_log.url
field since it has to be an URL within the site.
Jira: https://sourcegraph.atlassian.net/browse/CLOUD-18
CLOUD-18 #done