Improve overwriting shared layout - *BIC* notice.

There is a new template, `fluent_blogs/entry_contents_base.html`, which
defines the common appearance of entries in the archive and detail
pages. This makes overwriting entry layouts much easier. Standard Django
inheritance can be used, and the `{% if BLOG_LAYOUT == 'archive' %}`
trick is also no longer needed.

**BIC:** The `fluent_blogs/entry_detail/contents.html` template used to
override `fluent_blogs/entry_archive/item.html`. Now they are a common
template, `fluent_blogs/entry_contents_base.html`. The `BLOG_LAYOUT`
variable is gone.