Send message when a timeslot opens up.

What does this mr do?

Send a user updates on requests which have become available because of slots opening on the server side.

Screenshots

before

after

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made.
• Tests were created to test the feature.
• I have updated the documentation accordingly.
• I adhere to the style guide.

Closes #659 (closed) #569 (closed)