Add meta NOINDEX,NOFOLLOW to admin scope to avoid accidental crawling

[cmtickle]

Description

On occasion the admin url may be leaked to the frontend. This can (and has) result in the admin URL being available in search engines. This provides easy targets for brute force/password guessing hacks.

This fix will add a meta tag which instructs Google and other friendly bots not to add the admin URL to search results.

Manual testing scenarios

Visit the admin panel and view the source code. The head section should contain

<meta name="robots" content="NOINDEX,NOFOLLOW"/>

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[magento-engcom-team]

Hi @cmtickle. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
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[VladimirZaets]

Hi @cmtickle, please, sign CLA, otherwise, we can't process your pull request

[cmtickle]

Hi @VladimirZaets
I have signed the CLA today via the link provided. Thanks.

[VladimirZaets]

@cmtickle you should add the email address that you use for Github account to your git config.

[cmtickle]

Hi @VladimirZaets I've done that, CLA now showing as signed. Thanks.

[VladimirZaets]

@cmtickle Great, thanks

[magento-engcom-team]

@cmtickle thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.

[magento-engcom-team]

Hi @cmtickle. Thank you for your contribution.
We will aim to release these changes as part of 2.2.6.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.