Add configurable character limit for Service Desk subaddressing

Proposal

When using Service Desk custom email address we must ensure that the provider supports sub-addressing and we don't have any character limit on it at the moment. Some providers like AWS have 64 character limit in their RFCs so in cases that the email addresses exceed this limit, they get rejected/dropped.

We should have a parameter in the UI or gitlab.rb that should ensure that the sub-address substring does not exceed a certain limit.