css: Remove Bootstrap CSS reset for h5/h6.

This has no visual changes.

I hand-verified all the places we include these elements in the UI:
* Markdown rendered inside the app.
* Markdown rendered on portico pages.
* The only such element in our HTML templates is the <h5>s for
  organization logos.

For a couple of these, we were relying on the default margins provided
by Bootstrap.

static/styles/portico/markdown.css

@@ -78,6 +78,12 @@
         }
     }
 
+    h5,
+    h6 {
+        margin: 10px 0;
+        line-height: 20px;
+    }
+
     /* Since markdown doesn't make it easy to put an HTML element around a
        markdown table, we instead have a model of putting an empty div
        before it to configure a specific table's styling. */

static/styles/settings.css

@@ -1240,6 +1240,7 @@ $option_title_width: 180px;
         font-size: 1.2em;
         font-weight: normal;
         line-height: 1.2;
+        margin: 10px 0;
     }
 
     .sidebar-wrapper {

static/third/bootstrap/css/bootstrap.css

@@ -231,9 +231,7 @@ a.text-success:focus {
 h1,
 h2,
 h3,
-h4,
-h5,
-h6 {
+h4 {
   margin: 10px 0;
   font-family: inherit;
   font-weight: bold;
@@ -244,9 +242,7 @@ h6 {
 h1 small,
 h2 small,
 h3 small,
-h4 small,
-h5 small,
-h6 small {
+h4 small {
   font-weight: normal;
   line-height: 1;
   color: #999999;
@@ -268,12 +264,6 @@ h3 {
 h4 {
   font-size: 17.5px;
 }
-h5 {
-  font-size: 14px;
-}
-h6 {
-  font-size: 11.9px;
-}
 h1 small {
   font-size: 24.5px;
 }