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/base/020_base.scss

@@ -128,6 +128,13 @@ table tfoot {
   max-width: 70em;
 }
 
+.column-container {
+  display: flex;
+
+  .column-expand { flex: 1; }
+  .column-shrink { flex: 0; }
+}
+
 ul.list-bulleted {
   list-style: inside disc;
 }

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 {

app/javascript/src/styles/specific/keyboard_shortcuts.scss

@@ -1,16 +1,10 @@
-@import "../base/000_vars.scss";
-
 div#c-static {
   div#a-keyboard-shortcuts {
-    overflow: visible;
+    section.column {
+      min-width: 25em;
 
-    section {
-      min-width: 20%;
-      float: left;
-      margin-bottom: 1em;
-
-      h1 {
-        font-size: $h3_size;
+      ul {
+        margin-bottom: 2em;
       }
 
       li {

app/javascript/src/styles/specific/moderator_dashboard.scss

@@ -1,13 +1,6 @@
 div#c-moderator-dashboards {
-  div#col1 {
-    width: 45%;
-    float: left;
-    padding-right: 5%;
-  }
-
-  div#col2 {
-    width: 45%;
-    float: left;
+  #column-left {
+    margin-right: 2em;
   }
 
   div.activity {

app/javascript/src/styles/specific/posts.scss

@@ -331,10 +331,6 @@ div#c-posts {
       margin: 1em 0 0.5em;
     }
 
-    span.quick-mod {
-      float: right;
-    }
-
     .pool-name, .search-name {
       word-wrap: break-word;
     }

app/javascript/src/styles/specific/users.scss

@@ -4,16 +4,6 @@ div#c-users {
       margin-bottom: 2em;
     }
 
-    /* clearfix hacks */
-    div.box:before, div.box:after {
-      content: "";
-      display: table;
-    }
-
-    div.box:after {
-      clear: both;
-    }
-
     table.user-statistics {
       tr {
         height: 1.75em;

app/javascript/src/styles/specific/z_responsive.scss

@@ -77,7 +77,6 @@
   }
 
   article.post-preview {
-    float: none;
     margin: 0;
     text-align: center;
     vertical-align: middle;