Fix the assembly switch in inv-mv and add warning

A warning now appears when inv-mv is run on a part in an assembly. The
move is still performed.
Rename SR document assets to be more descriptive

latex-assets.zip being, IMO, a more descriptive name than data.zip.
Import Rich's spending.git api.

From spending.git 49f245d5.
Start writing a budget-line spending inspector.
Fix module path for budget loading from spending
Flesh out the search for a budget line
Print out transactions for a budget line.

The format for this is debateable. There may be a better way of
printing these.
Further fix up spending.py budget interaction
Adjust sp-line to fetch transactions+budget lines

Still using the existing api too
Generate a line summary
Spending lines: Print amount of payment pending
Zero pounds left in budget is underbudget
Make sp-line print the date as a string
Account for subdirectories in the spending API
Print budgeted/spent totals in sp-line.

This so that one can actually relate to how much we are over or under
budget, and how significant the pending amount is.
Remove a superfluous "pounds".
Add a script to make purchasing tickets and spending entries
Fix make_purchase to use the actual SR server

I'd accidentally left it using a debug setup, using my own machine
Add a script to add cheque numbers to spending transactions
Improve the accuracy of the items request in make_purchase