Using Flask-Caching, we can memoize the magnet_uri method. Here, a
timeout of 1 hour is chosen, though that value can be fiddled with.
The cache is defined in extensions.py, but gets initialised in
__init__.py.
* Replace all `from nyaa import app` imports with `app = flask.current_app` (or `from flask import current_app as app` where possible)
* Add a separate config object for top-level and class statements as `nyaa.extensions.config`
Required because those codes don't have app context at the time of evaluation/execution.
* Remove `routes.py` file and register all blueprints in `nyaa/__init__.py`
* Refactor `nyaa/__init__.py` into an app factory
* Update tools
* Update tests (temporary, will be replaced)