Bug 25520: Fix the SMS number input hint on the OPAC messaging page

Bug 25520: Fix the SMS number input hint on the OPAC messaging page

This makes the hint when entering an SMS number on the OPAC messaging
settings page the same as the staff interface hint: "Please enter
numbers only. Prefix the number with + or 00 if including the country
code." For some countries using either +XX or 00XX are accepted,
for example: +49 or 0049.

Test plan:
1. Set the SMSSendDriver system preference to Email
2. View the current hint for entering an SMS number in the staff
   interface:
   2.1 Go to Patrons > + New patron > Patron.
   2.2 Scroll down to the 'Patron messaging preferences' section at the
       end of the page.
   2.3 Note that the hint is "Please enter numbers only. Prefix the
       number with + or 00 if including the country code.".
3. View the current hint for entering an SMS number in the OPAC:
   3.1 Go to the OPAC > Your account (log in if required).
   3.2 Select the 'Messaging' tab/section.
   3.3 Note that the hint is "Please enter numbers only. Prefix the
       number with + if including the country code.".
   3.4 The difference: "..or 00.." is missing.
4. Apply the patch.
5. Refresh the OPAC messaging page.
6. The hint text for the OPAC is now the same as the staff interface.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>