webpage need some improvements

[LunaSare]

In downloads:

• give option to download a file with trees only

In graphs:

• branch lines are too thin
• x axis units should appear on all graphs. It's "Time (myrs)"

In plot of tree of median ages:

• letter size on timeline needs to auto-adjust to window size. If window is small, names overlap
• number size on timeline too
• numbers on timeline do not appear complete

In plot of all trees

• species names appear too small; size should auto-adjust

[LunaSare]

Done. See https://github.com/phylotastic/datelifeweb/issues/6