Consolidate validation of content

Previously we validated content outside of the content class via
calls to `is_valid_content` and some additional checks in page /
article generators (valid status).
This commit moves those checks all into content.valid() resulting
in a cleaner code structure.
This allows us to restructure how generators interact with content,
removing several old bugs in pelican (#1748, #1356, #2098).

- move verification function into content class
- move generator verifying content to contents class
- remove unused quote class
- remove draft class (no more rereading drafts)
- move auto draft status setter into Article.__init__
- add now parsing draft to basic test output
- remove problematic DEFAULT_STATUS setting
- add setter/getter for content.status
  removes need for lower() calls when verifying status

pelican/settings.py

@@ -126,7 +126,6 @@ DEFAULT_CONFIG = {
     'FILENAME_METADATA': r'(?P<date>\d{4}-\d{2}-\d{2}).*',
     'PATH_METADATA': '',
     'EXTRA_PATH_METADATA': {},
-    'DEFAULT_STATUS': 'published',
     'ARTICLE_PERMALINK_STRUCTURE': '',
     'TYPOGRIFY': False,
     'TYPOGRIFY_IGNORE_TAGS': [],