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

src/pretix/base/pdf.py

@@ -212,7 +212,7 @@ DEFAULT_VARIABLES = OrderedDict((
     }),
     ("addons", {
         "label": _("List of Add-Ons"),
-        "editor_sample": _("Addon 1\nAddon 2"),
+        "editor_sample": _("Add-on 1\nAdd-on 2"),
         "evaluate": lambda op, order, ev: "\n".join([
             '{} - {}'.format(p.item, p.variation) if p.variation else str(p.item)
             for p in (