chore(site-db-user): remove instances of old db user manager

[tanmoysrt]

• added archive_site_database_users function to call on site archive or moving to some unsupported site plan

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 39.68%. Comparing base (6b2f07f) to head (4d97a3f). Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
press/press/doctype/site/site.py	66.66%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2305 +/- ## ========================================== + Coverage 39.65% 39.68% +0.02% ========================================== Files 375 375 Lines 28610 28537 -73 ========================================== - Hits 11345 11324 -21 + Misses 17265 17213 -52 ```

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