Rename securedrop-admin to sdw-admin

[eloquence]

There's potential for confusion between our two uses of securedrop-admin: one installs servers or configures them, the other configures and provisions a workstation. Especially in a future where a Qubes laptop is used for server administration, this could cause issues.

We're using the sdw- prefix for some SDW-tooling like the preflight updater and notifier, so we agreed it could make sense to use it here as well, hence sdw-admin.