Remove admin ability not to use Hashed Storage

The administrator can enable and disable hashed storage for new projects, this defaults to hashed storage. However, given the extended usage of hashed storage already, and it being stable, it seems like a good idea not to even allow the admin to opt-out anymore.

This solves part of the split we're currently in with the legacy storage. It allows the Gitaly team to reason about storages more easily too. Now there's a couple security concerns that only arise if legacy storage is still enabled.

/cc @jramsay @fzimmer @brodock

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What needs to be done

• Modify the view and make Hashed Storage checkbox read-only when it is "enabled"
• Add a validation rule in application_settings to prevent hashed_storage configuration to be false
• Add a background migration to set hashed_storage setting to true