Resolve URL syntax at init/config time

Remove all the noise in the code about managing and adjusting the
Jira URL and centralise validation in two points:
- init
- configuration

Once the JiraConfig is resolved, all the rest of the code can assume
that the URL is valid and can be used as-is.

Change-Id: I0ff634216c8df5b60a897e67ff84aad980b56791