[57] singleUpload mutation uploads file to the database

[nicolaes]

What does it fix?

Closes #57

How has it been tested?

API tested with:

curl https://localhost:5001/graphql -k \
        -F operations='{"query": "mutation($file: Upload!) { singleUpload(file: $file) { fileBlobId } }", "variables": { "file": null }}' \
        -F map='{ "0": ["variables.file"] }' \
        -F 0=@room_sm.png

Or from Postman:

Migration tested with:

• Up: dotnet ef database update --startup-project TeacherWorkout.Api/ --project TeacherWorkout.Data/
• Down: dotnet ef database update AddImagesRelation --startup-project TeacherWorkout.Api/ --project TeacherWorkout.Data/

[nicolaes]

Contents of this PR already merged in #84