Account code regeneration major performance improvement

[adamcharnock]

I think this should drastically speed up the generation of full account codes.

The old:

• The old code would regenerate the the full account codes for all accounts
• It would do this upon any modification of the accounts table
• This made any account modifications something like an O(n**2) operation
• I clearly thought this was OK back in 2017, when my use case was well under 100 accounts!

The new:

• Now only children of the modified account are updated
• And even then only if the account code has been changed (postgres only)
• Any account codes of NULL or "" will cause all child full account codes to be NULL.
• I've added a couple more tests

Caveat:

• I've not ported this optimisation to mysql. @Joshun, if you're still around I'd very much welcome your assistance here (or anyone else for that matter). Lack of this wont block merging though. Update: done, grumbles.

And tagging @PetrDlouhy (#40), FYI.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.07%. Comparing base (81b90df) to head (390bd5c). Report is 6 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #117 +/- ## ========================================== + Coverage 93.01% 93.07% +0.05% ========================================== Files 59 61 +2 Lines 3836 3940 +104 Branches 249 256 +7 ========================================== + Hits 3568 3667 +99 - Misses 224 227 +3 - Partials 44 46 +2 ```

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[Joshun]

@adamcharnock thanks for this, I am planning on taking a look at this as soon as I have some spare capacity and will see how difficult it is to implement your improvements in the mysql version.

[adamcharnock]

Hey @Joshun! I realise now I didn't make this clear in my update – This did in-fact include a MySQL port of the changes. That being said I would certainly appreciate you looking over my work there as I haven't touched MySQL for quite a while.