Database Schema Updates for Sync Tracking

[dmitrytrager]

Add external IDs and checksums for sync tracking

• Add migration for topic_files:

add_column :topic_files, :external_id, :integer
add_column :topic_files, :external_checksum, :string
add_index :topic_files, :external_id, unique: true

• Add migration for topics:

add_column :topics, :external_id, :integer
add_index :topics, :external_id, unique: true

• Add migration for content_providers:

add_column :content_providers, :external_id, :integer
add_index :content_providers, :external_id, unique: true

• Add migration for tags:

add_column :tags, :external_id, :integer
add_index :tags, :external_id, unique: true

Do the same thing for whatever entities we would like to sync