css: refactor notice box css.

* Remove .ui-corner-all, .ui-state-highlight, .ui-state-error classes
  from notice boxes.
* Use .notice, .notice-info, .notice-error classes instead.
* Replace <p> elements in notices with <div>'s so that we don't have to
  work around the `margin-bottom: 1em` from <p> elements.
* Replace <h1> elements in notices with <h2>.
* Standardize info notices to use the same shade of light yellow in the
  light theme.

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

@@ -5,21 +5,6 @@ div#page {
   margin: 0 20px;
   padding: 0 10px;
 
-  div#upgrade-account-notice, div#ban-notice, div#dmail-notice, div#mod-notice {
-    margin: 1em 0;
-    padding: 1em;
-    text-align: center;
-    position: relative;
-
-    h1 {
-      font-size: $h2_size;
-    }
-
-    p {
-      margin: 0;
-    }
-  }
-
   aside#sidebar {
     width: 15em;
     float: left;

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

@@ -9,21 +9,42 @@ div.error-messages {
 }
 
 div#notice {
-  padding: 0.25em;
+  padding: 0.5em;
   position: fixed;
-  top: 0.5em;
+  top: 1em;
   left: 25%;
   width: 50%;
   z-index: 100;
 
-  &:not(.ui-state-error) {
-    background: var(--notice-background);
-    border: var(--notice-border);
-    color: var(--notice-text-color);
+  a#close-notice-link {
+    position: absolute;
+    right: 1em;
   }
 }
 
-a#close-notice-link {
-  position: absolute;
-  right: 1em;
+.notice {
+  border-radius: 3px;
+}
+
+.notice-small {
+  font-size: 0.8em;
+  margin: 0.5em 0;
+  padding: 0.5em 1em;
+}
+
+.notice-large {
+  margin: 1em 0;
+  padding: 1em;
+  text-align: center;
+  position: relative;
+}
+
+.notice-info {
+  background: var(--notice-info-background);
+  border: var(--notice-info-border);
+}
+
+.notice-error {
+  background: var(--notice-error-background);
+  border: var(--notice-error-border);
 }