Make theme more similar to Casper theme

Makes a number of tweaks and refactorings to make this hexo theme more
closely resemble [the Casper theme](https://github.com/TryGhost/Casper) ([this
page](http://lacymorrow.com/projects/casper/) used as reference).

Changes include:

- Adding next post / previous post links to the bottom of a post
- Reusing the existing site header for pages / posts
    - Disable the cover photo and change text from white to black on
      pages / posts
- Re-order comments section to *after* bio, sharing links
- Change how tags are listed (`| <tag>, <tag>` instead of `on <tag>
  <tag>`)
- Re-added curlicue (circle thingy) to top and bottom of articles