Have CommentStore limit the maximum character length of comments

Have CommentStore limit the maximum character length of comments

I92954c9 accidentally allowed comments, when CommentStore is enabled, to
be the full 65535 bytes available in the database field. This was never
intended.

There is not yet any consensus in T6714 or T6715 on just how long the
longer comments should be, for now we'll set 1000 because Tim says so.
Note this doesn't change the UI to actually allow more characters and
will only take effect once $wgCommentTableSchemaMigrationStage is raised
above MIGRATION_OLD.

Note this also doesn't make the limit configurable. That too can be done
later if needed, again along with whatever changes are necessary to the
UI to account for a variable limit.

Change-Id: I7e0c55619210ebab467436f0bb915271c0a7ccdc