Bug 29046: Change link in EmailFieldPrecedence and add link in EmailFieldPrimary

Bug 29046: Change link in EmailFieldPrecedence and add link in EmailFieldPrimary

This patch modifies the link in EmailFieldPrecedence to reflect the new
name of EmailFieldPrimary. It also adds a note in the latter to set the
'first valid' order in the former.

To test:
1. Go to Administration > Global system preferences
2. Search for EmailFieldPrecedence
3. Click on the link in the description
   --> It should lead to EmailFieldPrimary
4. Read the note in EmailFieldPrimary, make sure the grammar and
   spelling are correct.
5. Click the link to EmailFieldPrecedence
   --> It should lead to EmailFieldPrecedence

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>