Fix crash on very large orders

[raphaelm]

If an order with ~300 positions exist, even just fetching it from the database will crash our Android apps with something like:

2023-11-23 14:44:10.584  5926-6299  SQLiteQuery             eu.pretix.pretixpos.debug            E  exception: Row too big to fit into CursorWindow requiredPos=0, totalRows=1; query: select id, checkin_attention, code, deleteAfterTimestamp, email, event_slug, json_data, status, valid_if_pending from orders where code = ?

This PR stops storing the entire JSON representation of positions within the order itself, as we already have it in the individual row of the positions, therefore fixing the issue. Unfortunately, this introduces a likely performance regression in the cleanup of old subevents.

It will also require a change to some features in pretixPOS, for which I'll create a separate PR.

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (02945ed) 24.81% compared to head (c80b50f) 25.22%.
Report is 7 commits behind head on master.

Files	Patch %	Lines
.../java/eu/pretix/libpretixsync/sync/OrderCleanup.kt	0.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #32      +/-   ##
============================================
+ Coverage     24.81%   25.22%   +0.41%     
- Complexity      383      388       +5     
============================================
  Files           104      105       +1     
  Lines          5618     5807     +189     
  Branches        994     1024      +30     
============================================
+ Hits           1394     1465      +71     
- Misses         4005     4103      +98     
- Partials        219      239      +20     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.