Improve presentation of page navigation and #head area on mobile devices (#1742)

* Add `.header-title` style

This style shows smaller text on mobile devices. It reverts back to the
`2em` size used in Primer for H1 tags.

* Use `.header-title` for all `#head` headings

This commit:

  1. Updates all main templates to use the new `.header-title` class,
     making these titles less disruptively large on mobile devices.
  2. Centers these titles for mobile breakpoints only, re-setting them
     back to left-aligned text for `md-` breakpoints and up.

Note that in `lib/gollum/templates/wiki_contentf.mustache`, the
`.header-title` is not a `#head .header-title`, so it doesn't inherit
all of the new styles added in this commit.

* Adjust padding and presentation of mobile menu

To give mobile page headers more structure, this commit adjusts the
padding to be more symmetrical. It also adds a `border-bottom` on mobile
only. In my opinion this improves readability by telling the reader,
"This is where the navigation menu ends and the page content begins."

* Recompile assets

* Fix failing spec

My changes to the header layout seem to changed where the spaces in
`last_response.body` are placed. So let's assert only that the content
is present, since that's all this test should actually care about.

lib/gollum/templates/navbar.mustache

@@ -1,4 +1,11 @@
-<nav class="TableObject actions pt-4 px-2 px-lg-0 overflow-x-scroll">
+<nav class="TableObject
+            actions
+            border-bottom
+            border-md-0
+            p-2
+            pt-lg-4
+            px-lg-0
+            overflow-x-scroll">
   <div class="TableObject-item hide-lg hide-xl">
     {{>mobilenav}}
   </div>