Hot fix for Heroku

* Heroku uses DATABASE_URL env var.
* config.yml: Don't specify database for production when DATABASE_URL has
  assigned.
* days/config: use DATABASE_URL when config.database is not specified.
Remove duplicated semicolon
Improve /admin/entries page

* Make title column as link to edit page
* Add 'Show' button as link to permalink in Actions column
Improve /admin/entries/:id and /admin/entries/new page

* Show link to permalink right after "Editing Entry".
* Show published permalink in Permalink section of right column.

  * Renamed "permalink option" to "permalink"
Remove completed tasks
Add Days::Entry#short_rendered to implement "Continue Reading"
Implement 'Continue Reading'
Fix bug: last -> first
Query optimization for categories