o.c.opibuilder, beast.ui, trends.databrowser: Clear default email

When users send screenshots or alarms via email, they now need to enter
a "from:" email address. The default values, e.g. "opi@css", didn't tell
the recipient who sent the email, making it hard to reply.

Sites who want to use default 'From:' email addresses can add this to
their site-specific plugin_customiation.ini (as before).