Implementation of Request POST Methods

[Bera0422]

Implemented required methods to send collaboration/review requests and update request status. Unit tests are also implemented. This PR fixes #445

[hakanaktas0]

I believe you implemented these before the workspace class PR was merged. I think you should pull the backend again and revise your implementation.
One of the many issues I encountered: The collab request post api tries to insert to the database using 'id' field of the workspace table. The workspace table uses 'workspace_id' as its primary key.
It was really hard to make database function properly after the changes in the workspace class PR so please do not make migrations before pulling the latest version of the backend.

[Bera0422]

I believe you implemented these before the workspace class PR was merged. I think you should pull the backend again and revise your implementation. One of the many issues I encountered: The collab request post api tries to insert to the database using 'id' field of the workspace table. The workspace table uses 'workspace_id' as its primary key. It was really hard to make database function properly after the changes in the workspace class PR so please do not make migrations before pulling the latest version of the backend.

I pulled the changes from the "backend" branch and revised the id access to use "workspace_id". Unit tests are completed successfully.

[Bera0422]

Apparently, things went wrong. I will reopen the branch and the PR.