v8 - use ng-hide in validation directive

[nathanwoulfe]

Prerequisites

• I have added steps to test this contribution in the description below

Description

This is a left-over bit from work on #8894. In fiddling with that issue, I found that the showValidationOnSubmit directive used .show() and .hide() to control element visibility. That's all good until we want to change the visibility of the validation messages using the ng-show or ng-hide directive in a view. .show() and .hide() set an inline style attribute, which has higher specificity than the class added by ng-show or ng-hide so it's not possible to control the element visibility outside the events in validation directive.

To fix that, I've removed .show() and .hide() in favour of toggling the ng-hide class name on the outermost element. End result is the same (display property is set to `none!important' but it's done via a class rather than the style attribute, so isn't insanely specific. Also means that we're not mixing jQuery with Angular, which can (this PR as an example) cause unexpected behaviour.

[mikecp]

Thanks @nathanwoulfe :-)

[mikecp]

Woops did not see the review request prior to merging :-S

[umbrabot]

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