Bugzilla import: let user specify project key and lead

The bugzilla importer tries to guess an issue prefix from the Bugzilla project name. If the project names contain a common prefix ('Atlassian Foo'), the project keys will be quite meaningless ('ATL'). It would be better to let the user specify the key.

Also, it should be possible to set the project lead to something other than the id of the importer.