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

src/pretix/base/models/invoices.py

@@ -55,9 +55,9 @@ class Invoice(models.Model):
     :type footer_text: str
     :param foreign_currency_display: A different currency that taxes should also be displayed in.
     :type foreign_currency_display: str
-    :param foreign_currency_rate: The rate of a forein currency that the taxes should be displayed in.
+    :param foreign_currency_rate: The rate of a foreign currency that the taxes should be displayed in.
     :type foreign_currency_rate: Decimal
-    :param foreign_currency_rate_date: The date of the forein currency exchange rates.
+    :param foreign_currency_rate_date: The date of the foreign currency exchange rates.
     :type foreign_currency_rate_date: date
     :param file: The filename of the rendered invoice
     :type file: File

src/pretix/base/models/items.py

@@ -550,7 +550,7 @@ class Question(LoggedModel):
     :param question: The question text. This will be displayed next to the input field.
     :type question: str
     :param type: One of the above types
-    :param required: Whether answering this question is required for submiting an order including
+    :param required: Whether answering this question is required for submitting an order including
                      items associated with this question.
     :type required: bool
     :param items: A set of ``Items`` objects that this question should be applied to
@@ -667,7 +667,7 @@ class Quota(LoggedModel):
         again if those people do not proceed to the checkout.
 
     AVAILABILITY_ORDERED
-        This item is currently not availalbe for sale because all available
+        This item is currently not available for sale because all available
         items are ordered. It might become available again if those people
         do not pay.
 

src/pretix/base/models/orders.py

@@ -188,7 +188,7 @@ class Order(LoggedModel):
     def full_code(self):
         """
         An order code which is unique among all events of a single organizer,
-        built by contatenating the event slug and the order code.
+        built by concatenating the event slug and the order code.
         """
         return '{event}-{code}'.format(event=self.event.slug.upper(), code=self.code)
 
@@ -519,7 +519,7 @@ class AbstractPosition(models.Model):
     :type variation: ItemVariation
     :param datetime: The datetime this item was put into the cart
     :type datetime: datetime
-    :param expires: The date until this item is guarenteed to be reserved
+    :param expires: The date until this item is guaranteed to be reserved
     :type expires: datetime
     :param price: The price of this item
     :type price: decimal.Decimal