Added support for international phone numbers in volunteer model

[gotham13]

Issue Reference

This PR addresses the Issue : Fixes #841

Summarize

Please, describe what the PR does, the changes you have made.

Support for indian and international numbers
Added dependency django-phonenumber-field
Made changes in volunteer modal

[gotham13]

Just made changes for the volunteer model for now. Will change rest on furthur discussion

[atmb4u]

LGTM
Please verify this will NOT affect some of the dependencies on phone field which is already in production - once you can confirm on that, we can merge this to upstream.

[gotham13]

One thing that can break with this commit is + infront of international numbers . We need to verify wether it breaks the calling or sms feature. @atmb4u

[gotham13]

As + was present in the previous regex. I dont think it will break the app. This should be good @atmb4u

[shajith]

@Gotham13121997 I think this is a good idea in principle. Can you confirm what kinds of numbers are now valid/invalid? We had a nasty bug once that prevented data entry because of a wrong regex, just trying to understand what the change is.

[gotham13]

I have tested on indian numbers with 0 without 0 and with 91. and on international number that was mentioned in the issue. It seems legit. I have not done furthur checking on international numbers though. This was ported from libphonenumber by google so I trusted it should work

[KManiKumarReddy]

It's allowing alphabets d+420602123456 is allowing successful registration

[KManiKumarReddy]

Test against this list http://www.iaug.org/p/fo/et/thread=3486

[gotham13]

@KManiKumarReddy value going into database is a number though. I think it removes the alphabets by itself

[gotham13]