Enable link check add github token

[IgorTatarnikov]

linkcheck is now re-enabled.

Key updates:

• Enables passing the GitHub token to link check
• NiftyReg links changed from http://cmictig.cs.ucl.ac.uk/wiki/index.php/NiftyReg to https://github.com/KCL-BMEIS/niftyreg.
• The https://www.sainsburywellcome.org/web/people/sara-mederos is now dead, I've replaced it with a link to her orcid profile, but I'm open to other suggestions for a more permanent link.
• https://github.com/ramroomh doesn't exist as a GitHub account, I've removed the link to their card on the people page. Do we know of an up to date link to their profile?
• I added all sites/redirects that resulted in a 403 Forbidden to the linkcheck_ignore.

Closes #434

[adamltyson]

All looks good to me. I can't think of better ways to deal with these issues that what you've done. I won't merge yet just in case someone else does have a better idea.