* 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
* expand c4b184fa32
Mostly implement feedback by @iKevinY.
* rename content.valid to content.is_valid
* rename valid_* functions to has_valid_*
* update tests and function calls in code accordingly
