[ENH] Add scale bars to all plots

[oesteban]

All mosaics should contain scale bars (nilearn/nilearn#1743)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[effigies]

Reopening. Completing this will require fixing nipy/nipype#2791, since nilearn/nilearn@453d97f is only released in version 0.5.0.