Clean up and improve src\Plugin\Block\QuoraContent.php

Problem/Motivation

Follow up of https://www.drupal.org/node/2850804

Proposed resolution

1. Use dependency injection.
2. Move form from quora.module to block plugin.
3. Implement missing configuration functions.

Remaining tasks

1. Add category in block plugin annotation.
2. Use dependency injection to implement Drupal\Core\Routing\CurrentRouteMatch for getting current node context.
3. Move form code from quora.module to blockForm() function.
4. Implement missing defaultConfiguration(), getConfiguration() and setConfiguration() function for properly saving block form configuration value.
5. Remove _quora_content() and move code to build().
6. Use Quora Services.
7. Use $form_state->getValues() to avoid multiple calls to $form_state->getValue().