Bank sync/correct sorting

[bvanelli]

Fixes the incorrect sorting of the Actual transactions since the resolution of actualbudget/actual-server#556

Since the server and frontend code have been moved together, all the bank sync validations are now being done on backend, to allow support for more integrations. This means actualpy has to handle less logic when evaluating the bank sync.

This PR solves a couple of issues:

• Correctly calculates the starting balance for goCardless integration
• Correctly sorts the transactions by starting with the "Starting Balance", then consuming all sync transactions in reverse
• Correctly set the sort_order field by multiplying it by 1000 to convert to milliseconds
• Added Accounts.last_sync and Transactions.raw_synced_data. Nothing has been implemented for them

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.94%. Comparing base (ea640f2) to head (e4d18e8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #118   +/-   ##
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  Coverage   97.93%   97.94%           
=======================================
  Files          16       16           
  Lines        2329     2336    +7     
=======================================
+ Hits         2281     2288    +7     
  Misses         48       48           

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