Add env_management gem and .locals to gitignore

[fixbot]

Problem

We would like to switch to a pattern where we check-in .env.development and .env.test files and start using .env.*.local files for local overrides.

This change will require engineers migrate their local ENV vars. To help facilitate this change, we created a gem called env_management.

Solution

This PR replaces the dotenv-rails gem with stitchfix-env_management (which has a dependency of dotenv-rails). Once replaced, engineers can begin using env_management to migrate their local envs by running bundle exec migrate_local_env

Additionally, if the repo does not already list .env.development.local and .env.test.local in .gitignore, this PR will add them.

WARNING BEFORE MERGE

This PR stops dotenv-rails from being loaded in production. This PR is fine to merge UNLESS your app wrongfully has checked-in a .env file to load ENV vars in production.

If this app DOES have a checked-in .env, DO NOT MERGE THIS PR and notify #eng.

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[ ] This app does not have a checked-in .env file.