Add more DB configuration checks

[Łukasz Skarżyński]

In #91 we are introducing new Django check that validate system timeout settings on following database:

• postgresql -statement_timeout
• mysql - max_execution_time

The idea behind this group of checks is to help developers configure databases according to best practices.
If you have any ideas of such rules/checks, please shere in comments to this issue!

Some nice articles/sites about database configuration/settings:

• https://postgresqlco.nf/en/doc/param/