Smaller improvements (January)

A collection of smaller fixes and polish items:

• Tags on closed groups are now private. Non-members can no longer see a closed group's tag list; previously tag names leaked to signed-out or non-member visitors viewing a closed group's page.
• Group data exports exclude revoked memberships. HTML and CSV group exports now only include currently active memberships.
• Onboarding checklist relabeled. The sidebar "Try Loomio with your group" prompt is now called "Getting started checklist."
• Search modal wording. Cleaned up terminology in the search modal for consistency with the rest of the app.
• Comment editing reliability. Fixed a bug where editing an existing comment could intermittently fail to load its content, or fail to clear a draft correctly.
• Sign-up profile picture options. The option to use your SSO or social provider's photo was removed from the change-picture flow; Gravatar and manual upload remain.