emailField to stringField to allow multiple voicemail emails

[duduclx]

allowing to set multiple emails for voicemails notification.

so we can set emails separated by ","

this is a little regression as we don't use any regex to validate the emails.

[wazo-community-zuul]

Build failed.
https://zuul.wazo.community/zuul/t/local/buildset/a39770c2e2ff492298c28066a3bc9d12

❌ tox-linters FAILURE in 4m 05s
✔️ wazo-tox-py39 SUCCESS in 5m 11s
✔️ wazo-tox-integration-py39 SUCCESS in 4m 33s
✔️ debian-packaging-bullseye SUCCESS in 2m 37s

[DrPyser]

For this to make sense, wazo-confd /voicemails API and voicemail email notification logic would have to be updated to support multiple email addresses.
@duduclx if you have a related PR in wazo-confd please link it here as a Depends-On: field.