Resolve "Changes of childobjects (email, GPG, erfa) should also trigger an email"

Closes #104 (closed).

Also cleans up the code a bit and refactors large parts of the mailing code.

• Checking for changed values of Member and EmailAddress is done differently
• Always use templates for any kind of email.
• PEP8 conformity
• Some small code optimizations in several places
• Using Django's inlineformset_factory for simpler EmailAddress inlines in the Member Add form
• Set file encoding where needed
• Typos
• Minor bugs like there can be several, comma-separated email addresses in the email field when importing from the old database