Fix lingering technical debt issues from Timetracking v1.1

This issue is following up on unresolved comments here: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/901

Destructure import of prettyTime (const { prettyTime } = gl.utils;)
Update all props to have type defined to be able to use prop validation.
Double check default showHelp value for issuable-time-tracker