Add report to generate the "Cost of Doing Epic" poster

[amcewen]

We've long wanted to display the cost of running the space on a poster (just missed the opportunity to fix that on its decade-birthday!).

The reports @johnmckerrell created for the annual(-ish) blog posts have quite a bit of the stuff we'd need (income at least), we can add a new report to generate them.

[amcewen]

I've made a start on this today; adding a new cost_of_doing_epic action to the reports_controller. It doesn't do anything more than the "Income Distribution" report at the moment, but it's got a "last six months" button as it feels like the poster should cover more than just a month (maybe 3 months is a better period? Changing it will be pretty simple)

I think the next step would be to calculate the costs; then generate the graph from both; and then work out how to turn it into a PDF to print (possibly with weasyprint still).

[amcewen]

Have been chipping away at this.

It now collects the data and does some processing of it:

• Uses the incomings and outgoings from the bank entries, rather than from invoices (so incomings is based on actual money received rather than money we've asked for)
• It uses the same processing of the invoices as the "Income Distribution" page, and then uses that to split the actual income into more useful categories. So the total income amount is real, but the fact that £X is desks and £Y is mailboxes, etc. isn't completely true.
• It then further processes it to get a "monthly" (actually 31 days) amount for income and expenses.

Given it's to provide a guide, rather than a precise accounting, I think that "fuzzyness" is okay.

At present the generated HTML page is just a load of lists of amounts and categories that are useful in debugging. Next will be to turn the @monthly_outgoings and @monthly_incomings hashes into a chart.