add /tu/ (get) index

This commit implements the '/tu' Trusted User index page.

In addition to this functionality, this commit introduces
the following jinja2 filters:

- dt: util.timestamp_to_datetime
- as_timezone: util.as_timezone
- dedupe_qs: util.dedupe_qs
- urlencode: urllib.parse.quote_plus

There's also a new decorator that can be used to enforce
permissions: `account_type_required`. If a user does not
meet account type requirements, they are redirected to '/'.

```
@auth_required(True)
@account_type_required({"Trusted User"})
async def some_route(request: fastapi.Request):
    return Response("You are a Trusted User!")
```

Routes added:

- `GET /tu`: aurweb.routers.trusted_user.trusted_user

Signed-off-by: Kevin Morris <kevr@0cost.org>

aurweb/models/account_type.py

@@ -3,6 +3,11 @@ from sqlalchemy import Column, Integer
 from aurweb import db
 from aurweb.models.declarative import Base
 
+USER = "User"
+TRUSTED_USER = "Trusted User"
+DEVELOPER = "Developer"
+TRUSTED_USER_AND_DEV = "Trusted User & Developer"
+
 
 class AccountType(Base):
     """ An ORM model of a single AccountTypes record. """