Share URL is too long

For my monthly split (4 people and 24 expenses), the share URL is 3,554 characters long.

The current approach is:
1. Save the app state as a JSON string
2. Apply URL Encoding to percent-encode most special characters
3. Append that to the base URL in the `generalState` query parameter

I propose that we instead do this:
1. Save the app state as BSON data
2. Apply Base64 encoding
3. Append that to the base URL in the `b` query parameter (`b` for BSON, and `b` for second iteration)

We should _always_ accept the old approach, so that old URLs always work. However, we should default to this approach instead

Start by locating the share URL serialization and parsing that currently use the generalState query parameter. The work is done when new shares use BSON with Base64 in the b parameter while existing generalState URLs remain readable.

Written by the indexing model from the issue text.