[scorecards] Show comparisons between 2023 and 2025 Scorecards scores

[zarino]

TBC:

• Do we want to show comparisons for questions, sections, and overall score?
• Do we want to show comparisons on the "homepage" tables, or just the council pages?
• How do we want to handle newly merged councils? Council types might change too. Perhaps best to just avoid trying to "compare" newly formed councils.

Need to remember that questions will be changing between generations – will need a look-up for "what is the relevant 2023 question to compare to?"