Warn users about the removal of the themes other than mkdocs and readthedocs.

[d0ugal]

I was making this change, to start warning users about #201. We don't have a concrete plan yet but the earlier we start warning them, the earlier we can start making the move.

I didn't get very far with it, because It causes this error, which almost makes sense except that it doesn't at all...

======================================================================
FAIL: test_extra_context (mkdocs.tests.build_tests.BuildTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/dougalmatthews/Code/python/mkdocs/mkdocs/tests/build_tests.py", line 359, in test_extra_context
    self.assertEqual(config.validate(), ([], []))
AssertionError: Tuples differ: ([], [('theme', "The theme 'cosmo' will be remo[97 chars]s")]) != ([], [])

First differing element 1:
[('theme', "The theme 'cosmo' will be removed in an upcoming MkDocs release. See http://www.mkdocs.org/about/release-notes/ for more details")]
[]

+ ([], [])
- ([],
-  [('theme',
-    "The theme 'cosmo' will be removed in an upcoming MkDocs release. See "
-    'http://www.mkdocs.org/about/release-notes/ for more details')])

There are two big issues here.

• This test doesn't provide a theme, so it should use the default, mkdocs. The error is about cosmo.
• cosmo is only used in this test, which should be totally unrelated. Updating that test causes the one above to fail with the new theme.

So, there must be some leaky global state. I can't figure out where it is coming from. Ideas?

[d0ugal]

Managed to fix it, I don't quite understand what was wrong but tidying up the test sorted things out.

[landscape-bot]

Repository health increased by 0.18% when pulling f40e766 on d0ugal:wat into e616fe2 on mkdocs:master.

• No new problems were introduced.
• 1 problem was fixed (including 0 errors and 1 code smell).

[lbenet]

While I understand the rationale behind #201, it is worth to document how to include the bootswatch themes, which are already a separate package. This may even speed up the deprecation process for those themes.

[d0ugal]

Yup, that is a good point. It isn't documented because it doesn't work yet. It should be possible with the next release and then it will be documented. A release after that will remove the themes you see a warning about by default.