[change] Remove OPENWISP_EMAIL_TEMPLATE setting and recommend overriding email_template.html

[pandafy]

Is your feature request related to a problem? Please describe.
The OPENWISP_EMAIL_TEMPLATE setting does not take effect if an email extends the openwisp_utils/email_template.html template. This leads to confusion for users who expect the setting to apply universally, but it is bypassed when templates are extended directly.

Describe the solution you'd like
Remove the OPENWISP_EMAIL_TEMPLATE setting from the codebase. Update the documentation to clearly recommend users override the email_template.html template directly in the Django project if they wish to customize email templates.

We need to double check that overriding email template does not have any negative effect.