Move sign_icla helper into feature helpers
Remove hard-coded ICLA signer
ICLA Signers must have a linked GitHub account

This constraint is enforced in the IclaSignatureAuthorizer.

Note that the hook to reset the OmniAuth authentication stub for GitHub
now runs before each test case. This allows specs like the ICLA
signature feature spec to run in isolation.
Users must link a GitHub account to sign a CCLA
Fix comment in CCLA Signature controller
Format CclaSignatureController spec to match style
Add tests around post-account link behavior.
Document the require_linked_github! method.

The CclaSignaturesController and IclaSignaturesController have
different behavior when the user does not have a linked GitHub account.