Add parameter to email handlers not to add new users to the "JIRA Users" global permisisons

Jira 4.0 introduced user based licensing. Customers that use the 'createusers' option in one of their mail handlers have little control over the amount of users that their instance has. This is because every user created from the mail handler is assigned to the "JIRA Users" global permisisons, and therefore, they are granted permission to log into Jira, increasing the number of active users by one.

For customers that only want these users to interact by email (and therefore they do not need to log into Jira) it would be useful to have an extra parameter in the mail handler to indicate that the new user should not be granted "Jira Users" global permission.

To make sure these new users do receive mail notifications when the issue is updated, the browse project permission for the relevant project should be granted to group (anyone)