Add Snippet repository backfiling background migration

What does this MR do?

This MR adds the background migration that will call the background migration BackfillSnippetRepositories. That migration will create and initialize a repository to all snippets.

Since almost all snippets in the platform will need the migration, there won't be any problem regarding the id density per worker.

We also updated the migration BackfillSnippetRepositories because, when the users are blocked, they cannot perform any git operation. For those snippets, we will use a user admin to perform the migration for them.

In staging the migration of a snippet is around 1 second, and we have around 260k snippets in prod, so we will spend (((260000/100)*3)/60)/24 = 5.4 days

Related to #208904 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content