Migrate to Docusaurus (#329)

* docs: initial migration to docusaurus

* website: add custom font, update blurbs and icons

* website: update splash

* root: update links to docs

* flows: use .pbflow extension so docusaurus doesn't mangle the files

* e2e: workaround prospector

* Squashed commit of the following:

commit 1248585dca
Author: Jens Langhammer <jens.langhammer@beryju.org>
Date:   Sun Nov 15 20:46:53 2020 +0100

    e2e: attempt to fix prospector error again

commit 1319c480c4
Author: Jens Langhammer <jens.langhammer@beryju.org>
Date:   Sun Nov 15 20:41:35 2020 +0100

    ci: install previous python version for upgrade testing

* web: update accent colours and format

* website: format markdown files

* website: fix colours for text

* website: switch to temporary accent colour to improve readability

* flows: fix path for TestTransferDocs

* flows: fix formatting of tests

passbook/flows/forms.py

@@ -1,6 +1,7 @@
 """Flow and Stage forms"""
 
 from django import forms
+from django.core.validators import FileExtensionValidator
 from django.forms import ValidationError
 from django.utils.translation import gettext_lazy as _
 
@@ -62,7 +63,9 @@ class FlowStageBindingForm(forms.ModelForm):
 class FlowImportForm(forms.Form):
     """Form used for flow importing"""
 
-    flow = forms.FileField()
+    flow = forms.FileField(
+        validators=[FileExtensionValidator(allowed_extensions=["pbflow"])]
+    )
 
     def clean_flow(self):
         """Check if the flow is valid and rewind the file to the start"""