Harmonize markdown titles sizes

fixes #7757

closes #8029

Changelog.md

 # 0.7.12.0
 
 ## Refactor
+* Harmonize markdown titles sizes [#8029](https://github.com/diaspora/diaspora/pull/8029)
 
 ## Bug fixes
 * Improve handling of mixed case hostnames while fetching OpenGraph data [#8021](https://github.com/diaspora/diaspora/pull/8021)

app/assets/stylesheets/markdown-content.scss

@@ -16,6 +16,30 @@
     }
   }
 
+  h1 {
+    font-size: 2.7rem;
+  }
+
+  h2 {
+    font-size: 2.3rem;
+  }
+
+  h3 {
+    font-size: 2rem;
+  }
+
+  h4 {
+    font-size: 1.8rem;
+  }
+
+  h5 {
+    font-size: 1.6rem;
+  }
+
+  h6 {
+    font-size: 1.4rem;
+  }
+
   .img-responsive {
     display: inline;
   }