Allow users to download a zipped bundle of analysis results per dataset

[vladimirg]

(This is a continuation of the quota system - see #40.)

The zipped bundle should include:

1. The figures.
2. The GBrowse tracks.
3. A README containing an explanation about the files and instructions on citing Ymap.

This is useful if the user wants to delete older datasets to clear space (or just to remove their user from the system) but does want to keep the analysis results.