Clean up Web IDE global state data in QA specs

Issue: Clean up Web IDE global state data in QA specs (gitlab-web-ide#398 - closed)

What does this MR do and why?

This MR adds a shared context for Web IDE QA specs to:

• Clean up settings sync data after each context is run. This is done by calling the DELETE endpoint provided by the Settings Sync API. Note that this clears all Web IDE settings data for the user.
• Encapsulate repeated logic in the specs' before hook.

How to set up and validate locally

These instructions assume you are signed in the GDK with root user.

• Load Web IDE and change the IDE theme.
• Confirm changed setting is persisted: Run gdk psql command and then the following query:

select * from vs_code_settings where setting_type='settings' and user_id=1;

• You should see the theme you've set in the content.
• Close the Web IDE.
• In the gitlab directory, cd qa and run the QA spec with the following command:

QA_GITLAB_URL=<gdk_url> GITLAB_INTIAL_ROOT_PASSWORD=<password> bundle exec rspec <path_to_spec>

• Check the DB again: setting should be removed.