Add custom Markdown engine and blog UI changes

Blog UI changes in the admin, avoiding duplicate drafts and making it
easier to work on a draft blog post in two browser tabs (one for
updating the draft and another one for previewing it).

Fixes #177 and #139

Replace Github markdown service with an internal Markdown engine based
on Pegdown. Note that a custom VerbatimSerializer has been implemented
in order to write fenced code blocks compatible with the Google prettify
code highlighting library.

Fixes #473
Automatically create anchors in blog sub headlines

Create a custom Pegdown Serializer that outputs anchor links for each
subheadline in blog posts.

Fixes: #450
Enable pagination in blog admin dashboard

Fixes: #484