Pagination-aware issue listing

When we fetch issues before filtering them down per-week, we only get the first page, so may be missing issues.

https://github.com/CodeYourFuture/curriculum/pull/392 hacks around this by adding a proxy in the middle, but ideally I wouldn't be maintaining a proxy outside of the CYF world.

We should work out how we want to fix this properly, maybe with a Netlify serverless function written in JS/TS and hosted in CYF.