feat: project-level daily email sending rate

[supun-io]

• add a new column to save newsletter's daily_sending_rate, defaults to null. We will set the default limit in code.
• set default limit to 500 in code
• add a new input to newsletter settings to show the sending rate (it's not editable). Add a note to contact support to increase this.
• when sending, show a message like, "based on your sending rate, this issue will be sent over x hours (or x days)".
• update backend sending to distribute emails over the sending rate. If max is 500 per day, and the user is sending 1000, it will be equally distributed across 2 days. do the calculation
• update sudo /newsletters/:id to show the sending rate and also a way to update it.