Add tenant setting filter to Horde

It needs to take the hostname of the request, remove .internal (and handle any other cases there may be) to get the tenant identifier and set that on a ThreadLocal.

Implementing this will involve exploration of the libraries https://bitbucket.org/atlassian/mt and https://bitbucket.org/atlassian/atlassian-tenancy/, and translation of the implementation from the mt library into horde.

See: https://sdog.jira.com/browse/LEM-678