Create checkout system to prevent simultaneous coding

[elinw]

Qcoder is currently structured so that only one person can be coding a given project at a given time. However if two people have access to the same instance (or if the same person started two sessions) it would be possible for this to happen. We should prevent this.

One idea, when a project is selected in the shiny app temporarily rename the file(s) and then when qcode() is ended change the name back.