only display expand targets button when there is anything to expand to. Rename "expand" to "explain"
only display expand targets button when there is anything to expand to. Rename "expand" to "explain"

Reviewers: anupc

Reviewed By: anupc

Subscribers: changesbot, kylec, wwu

Tags: #changes_ui

Differential Revision: https://tails.corp.dropbox.com/D235493

dropbox/changes,dropbox/changes,dropbox/changes,dropbox/changes