Fix Rich Editor focus to fill the full height of the container (#4485)

Credit to @ayumihamsaki.

modules/backend/formwidgets/richeditor/assets/css/richeditor.css

@@ -523,6 +523,7 @@ body .fr-popup .fr-input-line textarea + label {background:transparent !importan
 body .fr-popup .fr-input-line input.fr-not-empty:focus + label,
 body .fr-popup .fr-input-line textarea.fr-not-empty:focus + label {color:#2a3e51}
 body .fr-popup .fr-checkbox span {border-color:#d1d6d9}
+.fr-element {height:100%}
 .field-flush .field-richeditor,
 .field-flush .field-richeditor.editor-focus {border:none}
 .field-richeditor {border:1px solid #d1d6d9;-webkit-box-shadow:inset 0 1px 0 rgba(209,214,217,0.25),0 1px 0 rgba(255,255,255,.5);-moz-box-shadow:inset 0 1px 0 rgba(209,214,217,0.25),0 1px 0 rgba(255,255,255,.5);box-shadow:inset 0 1px 0 rgba(209,214,217,0.25),0 1px 0 rgba(255,255,255,.5);border-radius:3px;-moz-border-radius:3px;-webkit-border-radius:3px;-moz-background-clip:padding;-webkit-background-clip:padding-box;background-clip:padding-box;transition:border-color ease-in-out 0.15s,box-shadow ease-in-out 0.15s;-webkit-transition:border-color ease-in-out 0.15s,box-shadow ease-in-out 0.15s;-moz-transition:border-color ease-in-out 0.15s,box-shadow ease-in-out 0.15s;-ms-transition:border-color ease-in-out 0.15s,box-shadow ease-in-out 0.15s;-o-transition:border-color ease-in-out 0.15s,box-shadow ease-in-out 0.15s}

modules/backend/formwidgets/richeditor/assets/less/richeditor.less

@@ -54,6 +54,14 @@
 @btn-active-text: #000;
 @input-label-color: @color-label;
 
+//
+// Make the focus ring and textarea fill the whole rich editor container
+//
+
+.fr-element {
+    height: 100%;
+}
+
 //
 // Rich Editor
 //