Replace Travis with GitHub actions and fix many typos (#1657)

* Create django.yml

* Fix working directory

* ..

* .

* ..

* a.

* ..

* .

* Fix typo

* Install hunspell

* maxfail

* Fix install

* .

* Reduce number of typos

* Even less typos

* Postgres debug

* Spelling fixes, yet again

* Postgres with PW

* Fix failing test

* New workflows

* Fix syntax error

* Install gettext

* Test aginst python 3.6 as well

* Clean up strategies

* Add badge, do not ignore migrations

* Use pip cache

doc/development/contribution/style.rst

@@ -18,7 +18,7 @@ Coding style and quality
 * We expect all new code to come with proper tests. When writing new tests, please write them using `pytest-style`_
   test functions and raw ``assert`` statements. Use `fixtures`_ to prevent repetitive code. Some old parts of pretix'
   test suite are in the style of Python's unit test module. If you extend those files, you might continue in this style,
-  but please use pytest style for any new test files.
+  but please use ``pytest`` style for any new test files.
 
 * Please keep the first line of your commit messages short. When referencing an issue, please phrase it like
   ``Fix #123 -- Problems with order creation`` or ``Refs #123 -- Fix this part of that bug``.