By eager loading these associations we can greatly cut down the number
of SQL queries executed when processing documents with lots of
references, especially in cases where there are references belonging to
the same project.

Since these associations are so specific to the reference parsing
process and the permissions checking process that follows it I opted to
include them directly in IssueParser instead of using something like a
scope. Once we have a need for it we can move this code to a scope or
method.