Finance Fixes - Slack in Thread Whenever an RR is Submitted to Sabo

Description

Blocked by #1615

Whenever a reimbursement request is submitted to sabo, we want to alert the user that submitted it (ideally in a public slack channel for visibility). This ticket is to send a message in thread of the original creation message in slack for the reimbursement request that @'s the submitter of the RR letting them know that their RR has been submitted to sabo.

If you wanna get fancy with this ticket you can also add emojis to the creation message to signify what state the reimbursement request is in

Acceptance Criteria

• A Slack message @ing the creator of the reimbursement request is sent in thread of the message in the finance channel about the creation of the reimbursement request whenever that reimbursement request is marked as 'Submitted to Sabo'

Proposed Solution

This will require a schema change, and understanding the slack code in the backend. Luckily this has already been done for change requests, and the slack ping for a user was done for pinging people on upcoming deadlines.

Start by reviewing the backend Slack code and the schema change required for reimbursement requests. Use the existing change-request Slack flow and the user ping for upcoming deadlines as reference points. Done means a threaded finance-channel message mentions the reimbursement request creator whenever its status becomes “Submitted to Sabo”.