feat(FastAPI): added /requests (get) route

Introduces `aurweb.defaults` and `aurweb.filters`.

`aurweb.filters` is a location developers can put their additional
Jinja2 filters and/or functions. We should slowly move all of our
filters over here, where it makes sense.

`aurweb.defaults` is a new module which hosts some default constants
and utility functions, starting with offsets (O) and per page values
(PP).

As far as the new GET /requests is concerned, we match up here to
PHP's implementation, with some minor improvements:

Improvements:

* PP on this page is now configurable: 50 (default), 100, or 250.
    * Example: `https://localhost:8444/requests?PP=250`

Modifications:

* The pagination is a bit different, but serves the exact same purpose.
* "Last" no longer goes to an empty page.
    * Closes: https://gitlab.archlinux.org/archlinux/aurweb/-/issues/14

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

aurweb/templates.py

@@ -71,6 +71,20 @@ def register_filter(name: str) -> Callable:
     return decorator
 
 
+def register_function(name: str) -> Callable:
+    """ A decorator that can be used to register a function.
+    """
+    def decorator(func):
+        @functools.wraps(func)
+        def wrapper(*args, **kwargs):
+            return func(*args, **kwargs)
+        if name in _env.globals:
+            raise KeyError(f"Jinja already has a function named '{name}'")
+        _env.globals[name] = wrapper
+        return wrapper
+    return decorator
+
+
 def make_context(request: Request, title: str, next: str = None):
     """ Create a context for a jinja2 TemplateResponse. """
 
@@ -83,6 +97,7 @@ def make_context(request: Request, title: str, next: str = None):
         "timezones": time.SUPPORTED_TIMEZONES,
         "title": title,
         "now": datetime.now(tz=zoneinfo.ZoneInfo(timezone)),
+        "utcnow": int(datetime.utcnow().timestamp()),
         "config": aurweb.config,
         "next": next if next else request.url.path
     }