Fixes #114

Regression was introduced by 45d4598e (Replace flask_oauthlib with plain
oauthlib). Support for HTTP Basic authentication scheme for client
authentication is actually required by RFC6749.

Fixes #115

Regression was introduced by 45d4598e (Replace flask_oauthlib with plain oauthlib).

flask_oauthlib is no longer available in Debian Bullseye. It is only a
wrapper around oauthlib, which is still available. While this change does
increase the OAuth2 code size, it achieves compatability with both Debian
Buster and Bullseye.

Aside from error handling, this change has no noticable effects on OAuth2.0
clients. In terms of error handling, a few cases that were not properly
handled before now return appropriate error pages.

Fixes #101

Add a few unit tests as well as integration tests that cover almost all views. Also fixed a lot of HTML validator errors.