css: switch mod dashboard, keyboard shortcuts pages to flexbox.

* Switch /static/keyboard_shortcuts from float-based layout to flexbox.
* Switch /moderator/dashboard from float-based layout to flexbox.
* Remove various .clearfix, `clear: both` rules.
* Remove dead .quick-mod rule.

app/javascript/src/styles/common/main_layout.scss

@@ -25,7 +25,6 @@ div#page {
 }
 
 footer#page-footer {
-  clear: both;
   width: 100%;
   margin-top: 1em;
   text-align: center;
@@ -33,10 +32,6 @@ footer#page-footer {
   border-top: var(--footer-border);
 }
 
-div.clearfix {
-  clear: both;
-}
-
 .sidebar-container {
   display: flex;
 

app/javascript/src/styles/common/paginator.scss

@@ -2,7 +2,6 @@ div.paginator {
   display: block;
   padding: 2em 0 1em 0;
   text-align: center;
-  clear: both;
 
   li {
     a {