Replace current_app.extensions-usages and proxies by access wrappers

[lukasjuhrich]

Rationale

Instead of using LocalProxys ourselves, we should just store the relevant objects in g and access that.
This would make the code easier to read and more in line with flask tutorials.

To make this well-typed, we could add a „type shim“ for g, which knows about the stored attributes, and import that.

Usages

sipa/sipa/backends/extension.py

Lines 340 to 341 in 004bb6a

	backends: Backends = cast(Backends,
	LocalProxy(lambda: current_app.extensions['backends']))

sipa/sipa/blueprints/register.py

Line 22 in 004bb6a

api: PycroftApi = LocalProxy(lambda: current_app.extensions['pycroft_api'])

sipa/sipa/model/sample/user.py

Line 38 in 004bb6a

config = LocalProxy(lambda: current_app.extensions['sample_users'])

sipa/sipa/model/pycroft/user.py

Line 25 in 004bb6a

api: PycroftApi = LocalProxy(lambda: current_app.extensions['pycroft_api'])

sipa/sipa/model/pycroft/userdb.py

Line 47 in 004bb6a

database = current_app.extensions['db_helios']

[lukasjuhrich]

This was based on a misunderstanding of the scope of the g proxy: It is lifetime-bound to the application context, however usually there is one application context per request.

Using app.extensions is fine, however for type-safe access one might go the flask_babel way and provide well-typed access methods like get_backends(app) / get_api(app).