Use update_bug_threshold access check for relationship add/delete

When adding, changing or removing a relationship between two issues,
both issues are modified. Therefore we need to ensure that the current
user has permission to modify both source AND destination issues.

The access threshold could be different for the source and destination
issues if they're both in separate projects. Therefore we need to get
the thresholds for each respective project and compare against each of
these thresholds.

This patch also removes hardcoded use of the default VIEWER access
level. Use of hardcoded default access levels is BAD because it breaks
custom access level configurations.