Various hordak 2.0.0 features

[adamcharnock]

My apologies for all this being in one PR, I know it isn't ideal. It is mostly fairly straightforward, except that a lot of lines were touched by the removal of django-model-utils. I'm going to get this merged now and then do the remaining changes hopefully in their own PRs.

Changelog

• BREAKING: New account currencies now default to DEFAULT_CURRENCY rather than all available currencies (trading accounts should be the only accounts with multiple currencies).
• BREAKING: Removed django-smalluuid. UUIDs in URLs will now be rendered in the regular UUID format.
• BREAKING: Removed use of django-model-utils. Model choices have the same values, but the data structures have changed to use the Django-native models.TextChoices:
  • Account.TYPES is now AccountType
  • TransactionCsvImportColumn.TO_FIELDS is now ToField
  • TransactionCsvImport.STATES is now TransactionCsvImportState
• New accounting-oriented database view on the Legs table (keep your accountants happier). Adds columns
  • debit - the debit amount, always positive, or null if the leg is a credit
  • credit - the credit amount, always positive, or null if the leg is a debit
  • amount - the amount, always positive
  • type - DR or CR
• Account code max length increased from 3 to 6

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.63830% with 54 lines in your changes missing coverage. Please review.

Project coverage is 92.05%. Comparing base (81b90df) to head (ef60492). Report is 15 commits behind head on master.

:exclamation: Current head ef60492 differs from pull request most recent head eb161a2

Please upload reports for the commit eb161a2 to get more accurate results.

Files	Patch %	Lines
...nagement/commands/create_benchmark_transactions.py	0.00%	51 Missing :warning:
...ak/management/commands/create_chart_of_accounts.py	75.00%	1 Missing :warning:
hordak/models/db_views.py	96.15%	1 Missing :warning:
hordak/utilities/currency.py	50.00%	0 Missing and 1 partial :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #124 +/- ## ========================================== - Coverage 93.01% 92.05% -0.97% ========================================== Files 59 64 +5 Lines 3836 4139 +303 Branches 249 261 +12 ========================================== + Hits 3568 3810 +242 - Misses 224 281 +57 - Partials 44 48 +4 ```

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