Move bank balance chart from site header to supporters section and supporters page

[equivalentideas]

Currently I think the bank balance chart in the header frames the site and project as being all about raising money—with is a tiny part of the project and not what we want to focus on. We do like the idea of the transparency of displaying our bank balance though, so rather than removing the chart, I think we should move it to places where it's more relevant.

This PR:

• removes the bank balance chart from the site header
• adds it to the donation section of the home page
• adds it to the top of the supporters page
• adds a title to the chart, "Our bank balance over time, updated 15/08/2017"
• extracts the chart from the header partial to it's own partial so it can be used more flexibly
• leaves the extra whitespace under the site title, I think it's nice 💐

See what you recon @e-e-e 😀 happy to change any part of this! 🤗

Before

After

[equivalentideas]

Thomke and Ben support the change in chat, so I'll merge this.