Add support for MySQL backend

[dvinegla]

Add support for MySQL backend
83448b7
was dropped in split migrations
b138fcb

[avelis]

The problem revolves around the query_params field with db_index set to True. There is a MySQL InnoDB restriction that text field based indices must have a defined length.

So, in theory you could either drop the db_index=True or specify a length parameter for the index to enforce.

I am in favor of dropping the index requirement.

[avelis]

Addressed in the following release: https://github.com/aschn/drf-tracking/releases/tag/1.0.1