Bug 24682: Make UsageStatsGeolocation readonly (and redirect to the config page)

Bug 24682: Make UsageStatsGeolocation readonly (and redirect to the config page)

To prevent invalid values in this pref (and so on the server), we should
make this input readonly on the syspref page.
The sysprefs related to Hea should be edited from the dedicated page, so
also adding a note about that.

Test plan:
Search syspref with "UsageStats"
Notice the note about the admin page
Notice the UsageStatsGeolocation input is now readonly (and resized, to
display the whole value when filled)

Signed-off-by: Maryse Simard <maryse.simard@inlibro.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>