**Background**
The Unblock API currently assumes `$wgEnableMultiBlocks` is false, and provides no means to remove a specific block.
For the Block API, see T378147
**Acceptance criteria**
* Accept a `blockid` parameter, which tells the backend to remove only that block
* Providing //only// a block ID is acceptable; You should not have to also specify a username.
** If both a blockid and username are provided, and there's a mismatch, error out with a detailed message. This is because the API is used for automation, and there's no way to require "confirmation" as we can for T378140 and T378144.
* If //no// blockid is provided, assume we want to remove all blocks (?)
** Doesn't seem safe for backwards compatibility. Maybe instead have a boolean flag to verify you want to remove all blocks? I.e. and a user id //is// provided, `removeall=1` all blocks for that user