Add env var description and fix URL

[munteanuic]

This PR is making the following changes:

• EXTERNAL_USER_TEAM in env variables will be unpopulated and will have a description placeholder instead
• The URL has a trailing '?' when all of the URL params become undefined. We'll only add the '?' if there are URL params
• The sharingLink passed to the iframe event shouldn't have any exploreId. We save links with exploreIds for the variable sharingExplorationLink
• Variable 'Final Sharing Link' is confusing since there is no Initial Sharing Link. In my opinion 'Sharing Link' is sufficient.