Implement automatic spell-check for docs (#688)

* [WIP] Implement automatic spell-check for docs

fixes #663

The only thing unclear to me so far is how the output of `make spelling`
should be checked - is there a possibility to check for a file (i.e.
`_build/spelling/output.txt`) to be empty, and report a failed build
otherwise?

* fix typo in requirements.txt

* add enchant library

* travis should report errors, order spelling wordlist

* change travis.yml to easier troubleshoot build issues

* fixed more typos, added more words

* add more words, fix more typos

* added more words

* added more words

* revert changes to .travis.yml

doc/development/implementation/settings.rst

@@ -35,7 +35,7 @@ Forms
 -----
 
 Hierarkey also provides a base class for forms that allow the modification of settings. pretix contains a
-subclass that also adds suport for internationalized fields:
+subclass that also adds support for internationalized fields:
 
 .. autoclass:: pretix.base.forms.SettingsForm
 
@@ -65,4 +65,4 @@ Plugins can add custom hardcoded defaults in the following way::
 Make sure that you include this code in a module that is imported at app loading time.
 
 .. _django-hierarkey: https://github.com/raphaelm/django-hierarkey
-.. _documentation: https://django-hierarkey.readthedocs.io/en/latest/
+.. _documentation: https://django-hierarkey.readthedocs.io/en/latest/