Add a different message for when all developers have submitted their timesheets

[chubberlisk]

Context

As spotted by @danielwilliams94 when all developers have submitted their timesheets, the message sent to the Learn Tech channel is a bit weird as it says These are the people yet to submit timesheets: and leaves it at that.

Changes proposed in this pull request

This PR adds message handling for when all developers have submitted their timesheets by using a different message for when this is the case.

It does so by updating the ShameLateDevelopersRequest boundary object to have two message attributes:

• ShameMessage (formally Message)
• NoShameMessage (newly added)

We then use either message depending on whether or not any developers have been received from the GetLateDevelopers use case.

With these changes, our CryptoTechReminderSystem.Main/Program.cs now expects a new environment variable called SLACK_NO_SHAME_MESSAGE. This will be eventually added onto Heroku.

Guidance to review

Might be easier to review by going through each commit separately.

Would like feedback on:

• The name NoShameMessage, it does the job but I'm not too sure.
• A new GetLateDevelopers stub was created called GetLateDevelopersEmptyStub, this was easier and quicker than reworking GetLateDevelopersStub to handle giving us an empty list. Do you agree?

Link to issue

#50 - When everyone has submitted their timesheets, the message is weird

[chubberlisk]

I think on a separate note, I think the phrasing of "shaming" goes against our no blame culture. We should consider the impact that the language we use, even in code, has on people.

@lukemorton Agreed, thank you for raising this. I'll merge this PR and open a separate PR to rectify this!