Repair DevTools button padding by centralizing styles

This moves the padding styles for dialog content to the .mx_Dialog rule. In
addition, it fixes vector-im/riot-web#7548 where the DevTools buttons had double
padding.

Signed-off-by: J. Ryan Stinnett <jryans@gmail.com>

res/css/_common.scss

@@ -170,8 +170,7 @@ textarea {
     font-weight: 300;
     font-size: 15px;
     position: relative;
-    padding-left: 58px;
-    padding-bottom: 36px;
+    padding: 0 58px 36px;
     width: 60%;
     max-width: 704px;
     box-shadow: 0 1px 0 0 rgba(0, 0, 0, 0.2);
@@ -216,14 +215,13 @@ textarea {
 }
 
 .mx_Dialog_content {
-    margin: 24px 58px 68px 0;
+    margin: 24px 0 68px;
     font-size: 14px;
     color: $primary-fg-color;
     word-wrap: break-word;
 }
 
 .mx_Dialog_buttons {
-    padding-right: 58px;
     text-align: right;
 }