Users confuse GitHub account linking with adding GitHub IDs to CLA

[ccascone]

Some users report that by linking their GitHub ID for member-only access, they expect that to be sufficient for validating CLAs on GitHub PRs.

The current UX where account linking is mixed with the CLA workflow is confusing. Perhaps we should separate the two (e.g., distinct page for account linking) and make it explicit that adding a GitHub ID to a CLA requires creating an addendum.