Fix i18n.extra_* spelling errors in documentation.

[torfsen]

Fix several instances in the "Configuration Options" documentation where ckan.i18n.extra_* was misspelled ckan.18n.extra_* (missing i), including in example source code.

Features:

• includes updated documentation
• includes user-visible changes

[rossjones]

Went to merge this, but then checked the circle/travis tests and they were moaning about the length of the underlines for some titles. Presumably they have to have an extra 2 whatevers added to them as well.

Really appreciate the fix, but would you mind awfully fixing up the underlines? Who knew documentation could be so fussy :(

The actual message was:

Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/case.py", line 331, in run
    testMethod()
  File "/home/ubuntu/virtualenvs/venv-system/local/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/ubuntu/ckan/ckan/tests/test_coding_standards.py", line 68, in test_building_the_docs
    "{warnings}".format(warnings=new_warnings))
AssertionError: Don't add any new warnings to the Sphinx build: 
['/home/ubuntu/ckan/doc/maintaining/configuration.rst:1609: WARNING: Title underline too short.', 
'/home/ubuntu/ckan/doc/maintaining/configuration.rst:1609: WARNING: Title underline too short.']

[torfsen]

No problem -- I deserve punishment for being too lazy to run the tests on such a small change. Props to the CKAN team for ensuring a clean coding style!

I've rebased and force-pushed to keep this in a single commit.

[rossjones]

What are circle and travis for if not to save you spending 20 minutes running tests for documentation ;)

Circle's happy, so merging. Thanks for the PR.