Add validators for complete and link call nodes

[kazuhitoyokoi]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Some nodes with validators have guided users to set appropriate settings. For example, an http-in node with an error triangle is easy to understand the lack of the URL path. To guide users in the same style, I added the validators for complete and link call nodes.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage: 68.554%. Remained the same when pulling 81331e6 on kazuhitoyokoi:master-validator into adfc5b3 on node-red:master.