[Analytics] UI fix to "Filter activities by date range" report

Had a quick discussion with Vicott on this and it appears to be confusing a lot of us:

To clarify how the date range works, it filters by the date of the activities made regardless of the date the player has joined, so the "Joined (GMT+)" column is actually a completely separate data point. 

However, to a lot of people, at first glance it seems like it would be correlated. Even though actually on closer inspection there is copywriting to indicate otherwise (second bullet point) but it's still not very clear.

Probably should restructure it so it's clear the two things are not the same?