Edit and delete messages that were scheduled

[ChaseC99]

Currently messages that were sent via Schedule Message are not included in the log. Therefore we can't edit or delete them on the /editMessage page.

This is slightly complicated to log because a message scheduled at (example) 1624940820 may not actually send until 1624940820.001500.

Proposal on how to handle this:

• A message to the log with the text, channel, scheduled ts, and a flag indicating that it is scheduled
• Every time we call loadLog(), if there is a scheduled message and we are past the scheduled send time, search the channel for a message containing the text around the scheduled ts.
• If the message is found, update the log message to match the normal message format with its actual ts. Else delete the scheduled message (if it's not there now, no reason searching for it later. Something had to have gone wrong....)

Edge cases to consider

• scheduled message is deleted before it's sent -> make sure to delete the log message
• scheduled message is edited before it's sent -> update the text of the log message