Merge branch '21395-standarize-markdown-headlines' into 'master'

Apply better hierarchy to markdown headers and issue/mr titles

## What does this MR do?
Applies better hierarchy to markdown headers and issue/mr titles

## Why was this MR needed?
Titles of issues/mr's were smaller than h1, leading to strange hierarchy when reading descriptions

## Screenshots (if relevant)

### Before:
![Screen_Shot_2016-10-14_at_1.31.37_PM](/uploads/79b6b16b8ed1b3b49578161eea1f0e42/Screen_Shot_2016-10-14_at_1.31.37_PM.png)

### After:
![Screen_Shot_2016-10-14_at_1.31.26_PM](/uploads/f710b63c112e342c5429810984079da6/Screen_Shot_2016-10-14_at_1.31.26_PM.png)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
Closes #21395

See merge request !6904

app/assets/stylesheets/framework/typography.scss

@@ -45,40 +45,38 @@
   }
 
   h1 {
-    font-size: 2em;
+    font-size: 1.75em;
     font-weight: 600;
-    margin: 1em 0 10px;
+    margin: 16px 0 10px;
     padding: 0 0 0.3em;
-    border-bottom: 1px solid $btn-default-border;
+    border-bottom: 1px solid $white-dark;
     color: $gl-gray-dark;
   }
 
   h2 {
-    font-size: 1.6em;
+    font-size: 1.5em;
     font-weight: 600;
-    margin: 1em 0 10px;
-    padding-bottom: 0.3em;
-    border-bottom: 1px solid $btn-default-border;
+    margin: 16px 0 10px;
     color: $gl-gray-dark;
   }
 
   h3 {
-    margin: 1em 0 10px;
-    font-size: 1.4em;
+    margin: 16px 0 10px;
+    font-size: 1.3em;
   }
 
   h4 {
-    margin: 1em 0 10px;
-    font-size: 1.25em;
+    margin: 16px 0 10px;
+    font-size: 1.2em;
   }
 
   h5 {
-    margin: 1em 0 10px;
+    margin: 16px 0 10px;
     font-size: 1em;
   }
 
   h6 {
-    margin: 1em 0 10px;
+    margin: 16px 0 10px;
     font-size: 0.95em;
   }
 
@@ -87,12 +85,12 @@
     font-size: inherit;
     padding: 8px 21px;
     margin: 12px 0;
-    border-left: 3px solid #e7e9ed;
+    border-left: 3px solid $white-dark;
   }
 
   blockquote:dir(rtl) {
     border-left: 0;
-    border-right: 3px solid #e7e9ed;
+    border-right: 3px solid $white-dark;
   }
 
   blockquote p {

app/assets/stylesheets/pages/detail_page.scss

@@ -20,9 +20,11 @@
 
 .detail-page-description {
   .title {
-    margin: 0;
-    font-size: 23px;
+    margin: 0 0 16px;
+    font-size: 2em;
     color: $gl-gray-dark;
+    padding: 0 0 0.3em;
+    border-bottom: 1px solid $white-dark;
   }
 
   .description {

app/assets/stylesheets/pages/merge_requests.scss

@@ -429,13 +429,6 @@
   }
 }
 
-.merge-request-details {
-
-  .title {
-    margin-bottom: 20px;
-  }
-}
-
 .merge-request-tabs {
   background-color: #fff;