theme: add robots meta tag to /signup and /login to prevent indexing

[GraemeWatt]

A meta tag should be added to the header of the /signup (register_user.html) and /login (login_user.html) pages like:

<meta name="robots" content="noindex,nofollow"> 

This will instruct web crawlers to not index the page and to not crawl any of the links on the page. I received a new notification today from Google Search Console that https://www.hepdata.net/oauth/login/cern_openid/ is indexed though blocked by robots.txt.

[GraemeWatt]

Also Sandbox records should not be indexed.