Billing management page

[timalces]

Aims to resolve #2. Dependent upon #1 being merged first.

• Adds a new page, billing management
• Shows a summary of the project's balance & budgets:
  • Remaining balance as of start of latest cycle
  • Costs so far this cycle
  • Estimated costs at the end of the cycle
  • Project's spend profile
  • Budget for the current cycle
  • Billing date
  • Project end date (if one)
• Shows a table listing project's billing history, containing the current and previous billing cycles
• Shows the date ranges, total cost and a link to the costs breakdown page for that date range
• Includes text indicating the current cycle and any cycles including estimated costs

Also adds a new navbar for moving between the different pages.

[timalces]

Merging this as seems to be working as intended.