Update emails

[jesseplusplus]

The default emails are a little confusing when paired with a named/skinned instance. To reduce confusion this PR:

• Removes "Mastodon: ...." from the subject line, so that the sender name and the subject aren't confusing to users (those who still want to use Mastodon as part of their name can set that up to display in the from email address)
• Adds an option in Admin > Site Settings to upload an email header logo that replaces the mastodon logo
• Removes references to public functionality and timelines, since this is a private-content-only fork