Add CompositeBlock class for enforcing multiple blocks

Add CompositeBlock class for enforcing multiple blocks

Create a CompositeBlock class which extends AbstractBlock and
adds the property $originalBlocks. This is for situations where
more than one block applies to a user/IP, and avoids the need
to choose just one of these blocks to enforce.

Behaviour of the resulting block is determined by combining the
strictest parameters of the original blocks.

Also add DatabaseBlock::newListFromTarget, which is similar to
DatabaseBlock::newFromTarget, but returns all relevant blocks,
rather than choosing the most specific one.

For tracking a CompositeBlock with a cookie, examine the
original blocks and only track the first trackable block that
is found.

Bug: T206163
Change-Id: I088401105ac8ceb2c6117c6d2fcdb277c754d882