Blocking could be checked against two tables (or even a table array). The
first one would be per-wiki, while the second one would be shared between
wikis.
Then, how would it be configured?
*Should any wikipedia admin be able to (un)block anyone?
*Should it be only done via meta / stewards?
*How would global-blocking be locally handled? Should local project have a
local-whitelist?
*An special page for it or integrated in the specialblock (a new checkmark
for example)?
*If we have blocks on both tables, with different expire time, which should
apply?