Set autofocus on Special:Unblock

Replaced the tabindex=1 with autofocus to have the cursor in the target
field. This is than also the start for tabs and therefor no tabindex is
needed.
In case of a valid block the autofocus is moved to the reason field,
because the target field gets hidden.

Change-Id: I1c35f546ad22907ab824b6c51fd65655f1a91a96
Merge "Set autofocus on Special:Unblock"