Emailer: Personalize emails with user info

[huang0h]

Why

Through the volunteer emailer, admins should be able to personalize the emails they send using the recipient's information from the table (pic)

What to do

• Update the SendEmailRequest DTO to take in user data in addition to their email (TODO update api spec with the shape of the data)
• Update the Emailer.sendArbitraryEmail() method to interpolate this user data into the email
  • We can use the same format that we use for our static emails - e.g. ${firstName} gets replaced with a user's first name
  • Leverage the code from EmailOperations.getTemplateString() to interpolate values - we should be able to abstract the logic from here