Introduce Workers::Mail::NotifierBase to be a base for all appropriate
mail workers to reduce code duplication

This functionality is not something we support and it doesn't look
it will be reintroduced soon.

see discussion https://github.com/diaspora/diaspora/pull/6818#discussion_r70227392

closes #7228

closes #7223

Fixes #7218, regression from #7036

closes #7219

closes #7222

closes #7221

closes #7224

closes #7217

closes #7209

closes #7213

closes #7201

closes #7208

closes #7210

closes #7202

fixes #6473

closes #7199

fixes #6969

closes #7198

closes #7186

fixes #5094

closes #7194

closes #6952