Handle local storage quota errors better

[dabreegster]

When we setItem, quota could be exceeded. At the very least, have a real error show up. Maybe auto-download the current file and instruct the user how to clear their cache. Or we could have an option to do that for them -- display the list of items in local storage, let them confirm and clear all to free up space.

We could also consider gzipping before storing (with a runtime penalty)

[dabreegster]

Ideas:

• first write a test to mimic running out of storage. Could use playwright to set some massive key slightly less than the limit to start, instead of trying to make up fake scheme data
• Catch the error with setItem. At absolute minimum, window.alert and explain the problem better
• If we can reasonably make a Modal appear when the problem happens, then having buttons to clear all local storage data or... even better, list out the keys, show the sizes, and let people delete each one to reclaim space
• We could also try compressing everything before we put it in local storage. I'd first check the runtime penalty to reading and writing. And then think through the backwards compatibility carefully; would definitely need some thorough tests before going this route

[dabreegster]

Another future idea is to investigate https://javascript.info/indexeddb

[dabreegster]

Overhauled in PR above