Bug 12802: Clarify system preferences

Bug 12802: Clarify system preferences

I felt the switch to multi in the EmailFieldPrimary preference was a bit
confusing given that type exposes a 'select all' option which doesn't
make sense with the pre-existing 'first valid' option being an override
in the code.

This patch opts to switch it back to 'Choice', meaning that only one
option can be picked and adds a 'selected addresses' option which
prompts the use of a new 'EmailFieldSelection' preference which allows
for the multi-select as before.

To test:
1) Run though the test plan for 'Update notice_email_address method to
   return a list' but with the following ammendments:

* 2) As aposed to being able to select multiple options under
  EmailFieldPrimary, you should now only be able to select one option at
  a time, but a new 'selected addresses' option should be present.
* 8) Select the 'selected addresses' option for 'EmailFieldPrimary' and
  also select multiple fields for the new 'EmailFieldSelection'
  preference.

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Axelle Clarisse <axelle.clarisse@univ-amu.fr>
Signed-off-by: Mathieu Saby <mathsabypro@gmail.com>
Signed-off-by: Aleisha Amohia <aleisha@catalyst.net.nz>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>