Add validator that applies all the rules

Problem/Motivation

Once the rule entity is created, #3495141: Add custom field for alt text validation rules to settings form add a validator that will run all the rules against the submitted value.

acceptance criteria

• do not perform validation if all validation is disabled
• process all the rules and return all of the compiled error notices, so that is does not just show the first error
• pass validation if only "warn" rules are triggered, but show a warning notification