Fix for Missing term error occurs in redmine trunk(4.2)

[y503unavailable]

Thanks for the useful plugin.

When using pivottable plugin in redmine trunk(4.2), an error without term occurs on the project summary screen.

In Redmine Core, these terms were deleted from the term file. https://redmine.org/issues/33342

label_overall_activity: Overall activity label_overall_spent_time: Overall spent time

As a measure for the following tickets, I copied the relevant terms from the language files of Redmine3.4