[Time Conductor] Validate numeric date-time fields

Allow injection of a validator for numeric values of
the date-time fields, to allow for these to be displayed
as invalid in cases where the text itself may be parsed
but the resulting value is not valid (e.g. when start
is greater than end.)

https://github.com/nasa/openmctweb/issues/325

platform/commonUI/general/res/templates/controls/datetime-field.html

@@ -24,7 +24,7 @@
     <input type="text"
            ng-model="textValue"
            ng-blur="restoreTextValue(); ngBlur()"
-           ng-class="{ error: textInvalid }">
+           ng-class="{ error: textInvalid, test: valueInvalid }">
     </input>
     <a class="ui-symbol icon icon-calendar"
        ng-if="structure.format === 'utc' || !structure.format"