Add custom error messages to blocked users for global and Tor blocks

The bug was about global blocks only, but the instructions apply to
Tor blocks as well. [[m:No open proxies]] has just been updated.

I'm leaving the links with Special:MyLanguage in the message because
the added markup is negligible and translators may want to customise
the target subpage in some cases: in some cases a target subpage may
better than the normal fallback, especially as unregistered users can't
use the UniversalLanguageSelector and Tor users will most likely not
have an account; updating links on translatewiki.net is easier than
in configuration but should not happen so often as to require a
separate message just for the link.

Requires the hooks to be added in the extensions:
Id429e01453bbb84692bf85486c59c973ef74da40
I447afd1fe56c5c8ac30a083d506d22a5a9119df4

Bug: 42231
Change-Id: Id45385f8dc3edaaef049c535ea6037c42e56ccd2
Merge "Add custom error messages to blocked users for global and Tor blocks"