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

website/docs/expressions/index.md

@@ -0,0 +1,69 @@
+---
+title: Expressions
+---
+
+Expressions allow you to write custom logic using Python code.
+
+Expressions are used in different places throughout passbook, and can do different things.
+
+:::info
+These functions/objects are available wherever expressions are used. For more specific information, see [Expression Policies](../policies/expression.md) and [Property Mappings](../property-mappings/expression.md)
+:::
+
+## Global objects
+
+-   `pb_logger`: structlog BoundLogger. ([ref](https://www.structlog.org/en/stable/api.html#structlog.BoundLogger))
+-   `requests`: requests Session object. ([ref](https://requests.readthedocs.io/en/master/user/advanced/))
+
+## Generally available functions
+
+### `regex_match(value: Any, regex: str) -> bool`
+
+Check if `value` matches Regular Expression `regex`.
+
+Example:
+
+```python
+return regex_match(request.user.username, '.*admin.*')
+```
+
+### `regex_replace(value: Any, regex: str, repl: str) -> str`
+
+Replace anything matching `regex` within `value` with `repl` and return it.
+
+Example:
+
+```python
+user_email_local = regex_replace(request.user.email, '(.+)@.+', '')
+```
+
+### `pb_is_group_member(user: User, **group_filters) -> bool`
+
+Check if `user` is member of a group matching `**group_filters`.
+
+Example:
+
+```python
+return pb_is_group_member(request.user, name="test_group")
+```
+
+### `pb_user_by(**filters) -> Optional[User]`
+
+Fetch a user matching `**filters`. Returns "None" if no user was found.
+
+Example:
+
+```python
+other_user = pb_user_by(username="other_user")
+```
+
+## Comparing IP Addresses
+
+To compare IP Addresses or check if an IP Address is within a given subnet, you can use the functions `ip_address('192.0.2.1')` and `ip_network('192.0.2.0/24')`. With these objects you can do [arithmetic operations](https://docs.python.org/3/library/ipaddress.html#operators).
+
+You can also check if an IP Address is within a subnet by writing the following:
+
+```python
+ip_address('192.0.2.1') in ip_network('192.0.2.0/24')
+# evaluates to True
+```