Simplify the login process

[thibaultamartin]

Steps to reproduce/test case

In order to log in on a Wallabag instance, the end user needs to generate manually a client_id and a client_secret, which are very long strings. He must then copy those very long strings in the login screen of the app, before entering his own credentials

Improvement

There is an issue opened on the Wallabag instance side to fix this. Especially, this solution proposal would fix the client_id and client_secret generation. It would be generated behind the scenes by the app and the instance. The user would only need to type in his instance URL, and his username and password.

This solution is not yet implemented, but I open this issue as a reminder to keep track of it.