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: archlinux/aurweb#14



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