* Don't raise exceptions when a BUR is invalid. Instead, use Rails
validations to return errors. Fixes invalid BURs potentially raising
exceptions in views. Also makes it so that each error in a BUR is
reported, not just the first one.
* Revalidate the BUR whenever the script is edited, not just when the
BUR is created. Ensures the BUR can't be broken by editing. Fixes a bug
where forum threads could be broken by someone editing a BUR and
breaking the syntax, thereby causing the BUR to raise an unparseable
script error when the forum thread was viewed.
* Validate that removed aliases and implication actually exist.
* Validate that the tag actually exists when changing a tag's category.
* Combine bulk update request processor unit tests with main bulk update
request unit tests.