Use DI in TfaUserLoginBlock

Problem/Motivation

Currently there's a call to \Drupal::config('user.settings') in TfaUserLoginBlock, that should not be there.

Steps to reproduce

Code inspection, line 104 of src/Plugin/Block/TfaUserLoginBlock.php

Proposed resolution

Replace it with a configFactory property of the block plugin class

Remaining tasks

Commit it.

User interface changes

None

API changes

We're alpha, baby!

Data model changes

None