Fix vertical alignment of GitLab SVG icons

- Add CSS to make GitLab SVG icons aligned by default
without the need to use extra utility classes like
`gl-vertical-align-text-bottom`.
- This has been applied to 12px and 16px icons since they
are the most common icon sizes.
- Remove existing re-alignment fixes that use position
relative.

app/assets/stylesheets/framework/buttons.scss

@@ -274,8 +274,6 @@
   svg {
     height: 15px;
     width: 15px;
-    position: relative;
-    top: 2px;
   }
 
   svg,

app/assets/stylesheets/framework/images.scss

@@ -48,4 +48,12 @@ svg {
       @include svg-size(#{$svg-size}px);
     }
   }
+
+  &.s12 {
+    vertical-align: -1px;
+  }
+
+  &.s16 {
+    vertical-align: text-bottom;
+  }
 }

app/assets/stylesheets/pages/tree.scss

@@ -138,12 +138,6 @@
   }
 
   .tree-item {
-    .file-icon,
-    .folder-icon {
-      position: relative;
-      top: 2px;
-    }
-
     .link-container {
       padding: 0;