Consolidate validation of content (#2128)

* 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

pelican/tests/test_contents.py

@@ -69,11 +69,13 @@ class TestPage(LoggedTestCase):
     def test_mandatory_properties(self):
         # If the title is not set, must throw an exception.
         page = Page('content')
-        with self.assertRaises(NameError):
-            page.check_properties()
-
+        self.assertFalse(page._has_valid_mandatory_properties())
+        self.assertLogCountEqual(
+                count=1,
+                msg="Skipping .*: could not find information about 'title'",
+                level=logging.ERROR)
         page = Page('content', metadata={'title': 'foobar'})
-        page.check_properties()
+        self.assertTrue(page._has_valid_mandatory_properties())
 
     def test_summary_from_metadata(self):
         # If a :summary: metadata is given, it should be used
@@ -503,7 +505,7 @@ class TestArticle(TestPage):
         article_kwargs['metadata']['slug'] = '../foo'
         article_kwargs['settings'] = settings
         article = Article(**article_kwargs)
-        self.assertFalse(article.valid_save_as())
+        self.assertFalse(article._has_valid_save_as())
 
     def test_valid_save_as_detects_breakout_to_root(self):
         settings = get_settings()
@@ -511,7 +513,7 @@ class TestArticle(TestPage):
         article_kwargs['metadata']['slug'] = '/foo'
         article_kwargs['settings'] = settings
         article = Article(**article_kwargs)
-        self.assertFalse(article.valid_save_as())
+        self.assertFalse(article._has_valid_save_as())
 
     def test_valid_save_as_passes_valid(self):
         settings = get_settings()
@@ -519,7 +521,7 @@ class TestArticle(TestPage):
         article_kwargs['metadata']['slug'] = 'foo'
         article_kwargs['settings'] = settings
         article = Article(**article_kwargs)
-        self.assertTrue(article.valid_save_as())
+        self.assertTrue(article._has_valid_save_as())
 
 
 class TestStatic(LoggedTestCase):