Django-pg-zero-downtime-migrations

- changed defaults for `ZERO_DOWNTIME_MIGRATIONS_LOCK_TIMEOUT` and `ZERO_DOWNTIME_MIGRATIONS_STATEMENT_TIMEOUT` from `0ms` to `None` to get same with default django behavior that respect default postgres timeouts
- added updates to documentations with options defaults
- added updates to documentations with best options usage
- fixed adding nullable field with default had no error and warning issue
- added links to documentation with issue describing and safe alternatives usage for errors and warnings
- added updates to documentations with type casting workarounds

- added django 2.2 support with `Meta.indexes` and `Meta.constraints` attributes
- fixed python deprecation warnings for regexp
- removed unused `TimeoutException`
- improved README and PYPI description

- added option that allow disable `statement_timeout` for long operations like index creation on constraint validation when statement_timeout set globally

- added long description content type

- replaced default sql queries with more safe
- added options for `statement_timeout` and `lock_timeout`
- added option for `NOT NULL` constraint behaviour
- added option for unsafe operation restriction