Transformed into an epic from: https://mattermost.atlassian.net/browse/MM-1000
Related to https://github.com/mattermost/platform/issues/643
Also request from @botherder: https://twitter.com/botherder/status/655394442313138176
Forum issue: https://forum.mattermost.org/t/how-do-i-set-the-base-url-for-apache-ssl-forwarding/573
1) For someone to host Mattermost in a subdirectory, it is assumed that Site URL is required to be set.
2) This change should not break existing deployments, including those who don't have Site URL set.