Explicitly treat Message plaintext parameters as plaintext

We have run into a few issues where user provided content passed
into a Message instance gets treated as wikitext in one way or
another. This patch is basically a followup to
I4da934dfe364acb0f87da1c8aed4a4d040d27a05 which was emergency
deployed last friday to deal with exactly this issue.

For the most part we have been solving for this with a combination
of Message::rawParams along with htmlspecialchars. This is less than
desirable and has lead to double encoding issues that have to be
carefully managed.

In I320645cd23c98fea4bfc32ab22b7ef8d320957cb for core I add a new
Message::plaintextParams and here we utilize it.  This gives proper
guarantees with regards to outputing the plaintext strings exactly
as we intend them to be output.

Bug: 66547
Change-Id: I5ae4d66b2d6099fb0e38a7073f766628d8560270
Merge "Explicitly treat Message plaintext parameters as plaintext"