Add mechanism to allow validation of metadata values for a given metadata
type.  Since a lookup table and validation are mutually exclusive, it is
possible to make the 'lookup' field in 'metadata_type' function for either.
This is accomplished by creating a new global settings field,
METADATA_AVAILABLE_VALIDATORS, which consists of a dictionary of
permissible validators.  apps/metadata/forms.py will check any entry
in the 'lookup' field of 'metadata_type' to see if it is a key in this
dictionary.  If it is, it is treated as a validator rather than a
generator of allowable entries.  A new method "clean_value" in forms.py
uses the validator function from 'lookup' to validate the 'cleaned_data',
and, if necessary, raise a ValidationError.
Remove SRMC specific information for settings and addition files