SpecialBlock [Codex]: guard against race conditions on initial load

In UserLookup, check against the existing list=allusers query to
validate the target instead of firing a separate API request to
list=users. For targets supplied by URL param, pass in blockTargetExists
and set that on the store's targetExists prop.

Validation within UserLookup is now confined to the single validate()
method.

Remove the blur listener which conflicts with the above strategy (i.e.
validate() could get called before the list=allusers query runs). The
blur listener shouldn't be needed, anyway.

This patch also moves the 'nosuchusershort' error to be on the
UserLookup field itself, instead of atop the whole form – something
Design and Product had requested from the beginning.

Finally, update module styles to reflect recent styling changes (such as
T383921) to reduce flashes of unstyled content.

Bug: T380970
Bug: T384712
Change-Id: I8172a13ffb26080b872bbf4b1a5a4f823acf9380

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