Add ManualSend class to back-send previous time ranges

[tdooner]

Due to an error, filings between 9/13 and 9/23 didn't go out as planned. This commit adds a way to back-send the filings for this range like so:

  send = ManualSend.new(
    date_from: Date.new(2020, 9, 13),
    date_to: Date.new(2020, 9, 23),
    notice: "Due to a server error, filings submitted between 9/13 and
      9/23 were not sent out via Open Disclosure Alerts. The error has
      been fixed and future filings will be sent out normally. The below
      email contains the filings during that timeframe. We apologize for
      the error and regret the delay."
    )
  send.preview
  send.execute!

• Update Filing to find filings within a date range
• Update AlertMailer to alter Email subject for a date range
• Add some tests to capture old and new behavior