Add admin check to ensure mbstring is loaded.

This ensures that Mantis is running at optimal performance.
Allow definition of view_bug_threshold

Following up on removing hard coded REPORTER references,(commit ref
2d605d6dda03e6ebd11348c6bedf532df8f78475) allow users to define a
view_bug_threshold to replace hardcoded instances of VIEWER.

This allows "hiding" projects from a user that would normally
have global developer access by setting the level to administrator only
for instance.