fix(advisory): adjust advisory publishing to new mailman hyperkitty s…

…etup

Extract the advisory generation functions to the advisory helper module
instead of the flask view. This allows an easier usage across different
views.

Instead of actually pulling the published content from the mailinglist,
lets store the current security tracker content instead.

fix: flask 2.2 and werkzeug 2.2 compatibility

fix(sso): adapt to authlib 1.0.0 behaviour

A undocumented change[1] in authlib 1.0.0.

[1] lepture/authlib#400

fix(feed): actually return proper atom including full content

Additionally, lets test for the appropriate content-type during tests.

chore(version): relax SQLAlchemy version constraints

SQLAlchemy-Continuum is compatible with SQLAlchemy 1.4 now and all other
incompatibilities are resolved.