Configurable Canonical URL

[ryanmark]

EXPLANATION

It might be necessary to include a canonical meta tag in the head of iframes generated by Vizier. If so, these should be configurable via the vizconfig file.

WHAT IS THE CURRENT BEHAVIOR AND HOW SHOULD IT CHANGE

Include a new field in the vizconfig and app settings. If the setting is populated, include the canonical tag in the iframe output.

WHAT IS THE MOTIVATION OR USE CASE FOR THIS NEW FEATURE

I'm not sure. Generally we do not want google to individually index iframe pages since people should not visit it directly. The canonical tag will prevent that. Ideally the canonical tag should point to the article where the graphic is embedded, but that requires the user to know and input the url for the published article, which is not always possible.