Bug 11926 [Follow-up] HEA - Render community koha statistic usages

Bug 11926 [Follow-up] HEA - Render community koha statistic usages

This follow-up makes some language and spelling corrections to the
system preference descriptions. I have updated the preferences heading
to read "Share anonymous usage statistics" in order to emphasize the
anonymous aspect.

I have also updated the main preference name from "UsageStatsShare" to
"UsageStats" so that it is alphabetized first in the list of
"UsageStats*" preferences. I think this will make it clearer to the Koha
administrator what feature they are configuring.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>