Implementing ccmdesign changes

[drkane]

From: https://github.com/ccmdesign/360-insights

Need advice/new styling from @ccmdesign:

• [x] Loading screen
• [ ] Fetch data modal (#52)
  • [ ] general design
  • [ ] progress bar (x2)
  • [ ] Button for when file has been fetched
• [x] Upload data button - needs current styles applied to default dash module
• [x] Chart styling #79
• [x] Filters - what to do when there's a long list
• [x] Multi-select dropdowns
• [x] Styling of subheaders & description next to charts #79
• [x] Map styling #79
• [x] Footer on chart page #56
• [x] Select a new dataset #66
• [x] years filter #71

To-do by @drkane:

• [x] Implement date range dash component - https://github.com/ThreeSixtyGiving/Insights/commit/530a543c20f42d7064b8125f7a67bf2bab2cb344
• [x] Blue stats bubble needs shorter numbers in the £1,000 - £1,000,000 range (use k suffix)
• [x] Make newsletter sign-up work