Improve layout of new & edit Entry forms
Automatically set Entry.content_html from Entry.content
Add Entry.is_page column

This will be used to distinguish "page" entries from non-page entries.
Create a home page and an entry in init_db script
Make a bunch of improvements to how entries/pages are displayed

- Add settings for the entries page title and URL.
- Allow entries to be accessed directly via /{slug} instead of only via
  /entry/{id}.
- Remove the Page model class. For now, Entry is used for both "entries"
  and "pages".
- Use 12-hour clock in format_datetime helper and strip leading 0s.
- Check for is_page when creating and editing entries.
- Show links for pages in nav bar (Entry.is_page == True).
- Fix <label>s in Entry create and edit forms (use IDs, not names).