Migrate import log to new log system

Authored by Umherirrender.


Migrate import log to new log system

There is no need for an extra formatter class, because at the moment
there is nothing extra to format.

This allows use of gender on Special:Log. Old messages are kept for use
in IRC. A test was added to ensure an unchanged IRC message.

The new log message for import-interwiki is used from
I3ca8b21bce49b41cac7109efb8056ca4469b88d7 to avoid "transwiki jargon".

Bug: T57404
Change-Id: I590075b203548c071a98c1eaa4b1bc9250f60e22