OTP-Token emails don't contain anything besides the token

Currently, these emails look like this:

• Address the specific user to make the email look less like spam
• Style it somehow
• Still have the token as a big bold text in a prominent location
• Explain, what this token is, what to do with it (ggf link), what to do if this token wasn't requested, and so on
• Maybe use the site logo somewhere
• Definitely use the name of the aleksis instance somewhere to indicate the source of the email