Fix #875 validation in MY_Model
Fix #900 & added field metadata to model & user

- Added field metadata support to BF_Model, including:
- $field_info property to cache field data (can be hard-coded in child
models as desired)
- get_{created/modified/deleted}_field() methods
- get_{created/modified/deleted}_by_field() methods
- get_field_info() method
- prep_data() method to prep post data if fields in post data match
names in field_info
- Updated user_model and settings controller in users module to use the
updated functionality in BF_Model as a partial example.
- Added force password reset checkbox to individual user settings view