Support excluding all fields from having database indices in the historical table by using `__all__` in `no_db_index`

[rajarehanahmed]

Problem Statement
Although we can specify individual fields with db_index=True in no_db_index to exclude them to be indexed in the historical model, there's currently no way to exclude all indexed fields without listing each one explicitly.

Describe the solution you'd like
A simple sentinel value __all__ that can be used in no_db_index to exclude all fields from being indexed.

Describe alternatives you've considered
We are going to inherit the simple history model and override the functionality in our own codebase till this goes live.

Additional context
None

[rajarehanahmed]

@ddabble could you please have a look at this issue and the PR, thanks.