Remove old migrations

What does this MR do and why?

In this MR, we are squashing migrations of the year 2019.

I have decided to only squash migrations for this year because we have a limitation in the number of files that we can change/remove. I will open a new MR for the year 2020.

I have used the following script to be able do squash the migrations and remove the spec files.

1. bundle exec rake db:drop db:create db:migrate VERSION=20190909141517 -> Drops the database; Creates a new database; Migrates to a specific version
2. bundle exec rake db:structure:dump SCHEMA=db/init_structure.sql -> Creates a new structure file with the name init_structure.sql
3. rake remove_old_migrations[20190909141517] -> Removes all migration files + specs (*) MR
4. bundle exec rake db:drop db:create db:migrate -> Creates a new database without using the old migrations

(*) We are not removing the init schema migration.

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How to set up and validate locally

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Related to #331248 (closed)