Validation constraint added for workgroup

Added validation constraint on `value` for **Workgroup**. In follow up of [Bug #1079](https://bugs.amahi.org/issues/1079).
Workgroup must have:
- maximum length of 15
- should not contain any special characters

For this, I am validating the `value` for both these conditions by - `if` that record is of `kind` 'general' and has `name = workgroup`. On updation of such record, this validation is triggered.