[lib] introduced messageContentForServerDB function in reaction message spec

Summary:
introduced messageContentForServerDB function in reaction message spec. messageContentForServerDB returns the targetMessageID and the reaction as a stringified JSON object. The contents of this function are actually the same as messageContentForClientDB, so messageContentForClientDB can actually just call and return messageContentForServerDB for it's method now.

UPDATE

Based on some feedback I got on some subsequent diffs, I have removed targetMessageID from the messageContent in messageContentForServerDB. The big change with this update is that we should now be pulling targetMessageID directly from the server row instead of content. We are now separating the two messageContent functions since rawMessageInfoFromClientDB still needs targetMessageID from the client and ClientDBMessageInfo doesn't have a target_message_id field

Depends on D5690
Linear Task: ENG-2245

Test Plan: Ran web and mobile app locally and nothing crashes/will be doing more tests on this message spec in subsequent diffs

Reviewers: atul, rohan, tomek

Reviewed By: atul, rohan, tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5698