[#2130] Fixed OAuth OOB authentication for users with enabled MFA

Closes #2130

See merge request pleroma/pleroma!2979

[#2497] Media preview proxy: misc. improvements (`static` param support, dynamic fifo pipe path), refactoring.

Refactored login code in OAuthController, reused in AccountController. Added tests.

Unifies all the similar functions to one and simplify some blocks with it.

Maksim authored May 07, 2020 and lain committed May 07, 2020

Pleroma.Web.TwitterAPI.TwoFactorAuthenticationController -> Pleroma.Web.PleromaAPI.TwoFactorAuthenticationController

Enforcement of OAuth scopes check for authenticated API endpoints

See merge request pleroma/pleroma!2349

[#2409] Tested all auth setup configs in AuthTestControllerTest. Adjusted :skip_plug definitions for some endpoints.

Enforcement of OAuth scopes check for authenticated API endpoints, :skip_plug plug to mark a plug explicitly skipped (disabled).

Done via the following command:
git diff fcd5dd25 --stat --name-only | xargs sed -i '/Pleroma Authors/c# Copyright © 2017-2020 Pleroma Authors <https:\/\/pleroma.social\/>'

[#1478] OAuth admin tweaks: enforced OAuth admin scopes usage by default, migrated existing OAuth records. Adjusted tests.

[#963] No redirect on OOB OAuth authorize request with existing authorization. OAuth-related refactoring.