Resolve "Turn Step into an Active Record model stored in the DB"

In the scope of this MR 2 major (but connected with each other) elements were moved to the DB:

1. steps are active model stored in DB now
2. DataFile has DB reference to DataFileType. Previously enum was used, so we were not able to easily add new data file type without modifying the codebase

You can also notice that some elements are not removed from the DB (e.g. data file type enum or pipeline_step field). These data are needed to migrate existing data to a new schema on development and production. In the next release, I will create cleanup MR where these elements will be removed as well as migration tasks.

On development and production these tasks need to be invoked after the push:

1. rails data_files:connect_with_type
2. rails steps:eurvalve:seed
3. rails flows:eurvalve:steps

Closes #390 (closed)