Mechanism for adding details of where numbers come from

[sarahbridle]

e.g. add hyperlink to a spreadsheet which contains all the default numbers, with a column containing 'more info' - so you can type as much info as you want about the references into the relevant cell

[jucordero]

I've partially addressed this in #23, by adding a button to display a modal with links to relevant references. There are still some unreferenced numbers in the defaults for the advances settings.

In the meantime we can create a spreadsheet with all references and link it in the dashboard, but I would really like to see a direct link to the original source being displayed in the dashboard itself, as intermediary google docs add a point of failure.