Support RobotsTxt module

Problem/Motivation

RobotsTxt module (/robots.txt) does not work correctly if default site language is not allowed.

Steps to reproduce

Install RobotsTxt, add another language and disallow access to English (site default)

Proposed resolution

Add 'robots.txt' to pathIsWhitelisted function in LanguageAccessSubscriber.php OR whitelist routes that use _disable_route_normalizer: 'TRUE'

Remaining tasks

User interface changes

API changes

Data model changes