policies/expression: migrate to raw python instead of jinja2 (#49)

* policies/expression: migrate to raw python instead of jinja2

* lib/expression: create base evaluator, custom subclass for policies

* core: rewrite propertymappings to use python

* providers/saml: update to new PropertyMappings

* sources/ldap: update to new PropertyMappings

* docs: update docs for new propertymappings

* root: remove jinja2

* root: re-add jinja to lock file as its implicitly required

docs/expressions/index.md

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+# 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")
+```

docs/expressions/reference/user-object.md

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+# Passbook User Object
+
+The User object has the following attributes:
+
+ - `username`: User's Username
+ - `email` User's E-Mail
+ - `name` User's Display Name
+ - `is_staff` Boolean field if user is staff
+ - `is_active` Boolean field if user is active
+ - `date_joined` Date User joined/was created
+ - `password_change_date` Date Password was last changed
+ - `attributes` Dynamic Attributes
+
+## Examples
+
+List all the User's Group Names
+
+```python
+for group in user.groups.all():
+    yield group.name
+```