Allow the CSV importer to re-use a user based soley on the fullname.

For example, if an Assignee in the import (e.g. John Doe) matches a Full Name in Jira (e.g. John Doe), Jira should choose that pre-existing user for the Assignee.

At present, the CSV import will expect the username to be a lowercase concatenation of the fullname or a lowercase first initial concatentation with lastname based on the mapper that is used.

For example, if a user with username jdoe and fullname John Doe exists within JIRA - the CSV import will create a new user with username johndoe and fullname John Doe. In this case, the pre-existing user should have been used.