Established ::placeholder selector style

The default `::placeholder` styles—at least in Chrome—were really close to our default color for `input` fields, and thus difficult to distinguish. Overwriting this as `$grey-lighter` resolves that ambiguity, making it clearer what's an explicit value and what's an inherited value.

There's also a `::-ms-input-placeholder` vendor prefix which is _supposed_ to be used, but if that's included then none of the styles are honored. It's unclear why that is, and it seems like a potentially serious bug in how CSS is processed. But since most of our customers aren't using Edge, it's not a priority to fix.

Also, I needed to use `!important`. Unfortunately, Google Chrome—at least—doesn't seem to show `::placeholder` via the debugger console, even though it shows other pseudo-elements, so it's unclear what value is taking priority over this. We certainly aren't defining `::placeholder` anywhere else in our `*.scss` implementation.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Shared/Styles/Base/_forms.scss

@@ -162,4 +162,12 @@ section.attribute input.error {
   &.error {
     color                       : $red;
   }
+}
+
+//----------------------------------------------------------------------------------------------
+// PLACEHOLDERS
+//----------------------------------------------------------------------------------------------
+::placeholder,
+::-webkit-input-placeholder {
+  color                         : $grey-lighter !important;
 }