Allow using GivenName and SN attributes in LDAP config

Nowadays it's only possible to use the displayName attribute in FishEye.
However, other Atlassian products such as Bamboo allow using givenName and sn attributes instead.

I've just came across this use-case:

• In LDAP, all hundreds of users' displayName attribute is in the {lastName, firstName} format. These users also have givenName and sn attributes configured accordingly.
• In FishEye, displayName is being used in Display Name Attribute field.
• However, now in FishEye I want to display user names in the {firstName lastName} format.
• Changing all hundreds of users's displayName in LDAP and re-sync the user base in FishEye is definitely not practical, it would require a lot of manual work.
• FishEye should allow using the givenName and sn attributes along with displayName attribute, just like other Atlassian products do.