Deprecate User::isBlocked()

Deprecate User::isBlocked()

The method User::isBlocked() attempts to answer two questions:
(1) Does the user have a block?
(2) Is the user prevented from performing this action?
The method can answer #1, but it cannot answer #2. Since User::getBlock() can
also answer #1, this method is redundant. The method cannot answer #2 because
there is not enough context in order to answer that question.

If access is being checked against a Title object, all access checks can be
performed with PermissionManager:userCan() which will also check the user's
blocks.

If performing all access checks is not desirable, using
PermissionManager::isBlockedFrom() is also acceptable for only checking if the
user is blocked. This method does *not* determine if the action is allowed,
only that the user's block applies to that Title.

If access is being checked without an existing Title, User::getBlock() can be
used to get the user's block. Then Block::appliesToRight() can be used to
determine if the block applies explicitly to a right (or returns null if
it is unknown or false if explicitly allowed). If the user is creating a new
Title, but the text of the title is not yet known (as in the case of Wikibase),
access should be checked with Block::appliesToNamespace().

Bug: T209004
Change-Id: Ic0ad1b92e957797fee8dcd00bd1092fe69fa58f1