Add in validation to disallow forward slashes or other illegal chars in Manufacturer and Model fields

[duncangreene]

What is the problem your feature is trying to solve?
Add in form validation to disallow forward slashes or other illegal chars in Manufacturer and Model fields, as these are used in the directory path. It is currently possible to add in forward slashes, but will error out when the 'Save' button is pressed.

Potentially related to #1093 and #1094.