Feature summary: Support partial global blocks for account creation only.

Use case: Global blocking previously only allowed to choose between anon-only or hard blocks, both options preventing account creation. T17273 added an option to allow account creation when globally blocking IPs / IP ranges.
As an admin I sometimes use partial range blocks to prevent local LTA from creating sockpuppets, but don't block IP editing and instead use an abuse filter to only prevent specific IP edits in order to mitigate the impact for good faith anon-editors using the same IP range. As a steward I would like to get the same option for global blocking (T17273#10066969 / T373367#10093378)

Benefits: Feature parity with local blocks, mitigate the damage of range blocks.