report tree retrieval separate from rendering

[arlin]

The stress test today reveals that the portal sometimes fails to render a tree, or delays for a very long time. During this time, the user actually can access the tree using Export as, but the user does not know this. The scaled trees may become accessible also. The tree retrieval message does not appear until after the tree has been rendered.

• Notify the user as soon as the tree is retrieved.
• Update the notification message to indicate that the tree is being rendered. "A tree with X tips has been retrieved. You can get the Newick string with Export as: Newick. A graphical view will appear below."
• Update the tip count for the unscaled tree as soon as the unscaled tree has been retrieved. Do not use ETE to count the tips. Use DendroPy or something else.