Fix wrong language constants for quickmod/modcp actions

There are some language constants in quickmod/modcp that do not correspond with their respective strings.

FORK_TOPIC seems to have been around forever; this action is called copying a topic in UI.

PHPBB3-9630 adds merge posts/merge topic constants to the mix which should be called move posts instead.

In 3.1 the constant names should be changed to reflect their text.