[Feature Request] enhanced email marketing

[g00g1]

Is your feature request related to a problem? Please describe. There is Mass Mail Tool extension at FOSSBilling, but capabilities of the Recipients filter are incredibly low - there is even no possibility for filtering clients that opted into email marketing.

Describe the solution you'd like I think two things could be done:

1. add possibility to change custom field type (currently it is string)
2. add optional filter for Mass Mail Tool which checks selected custom field for true/false

Describe alternatives you've considered Currently the only alternative I am aware of is usage of external means, for example something like public read-only mailing list

Additional context null

[Anuril]

You're right, there needs to be changes, also to comply with GDPR. I'll look into it and update the module.

[j-a-pope]

Related to #254