Allow measure expressions to be visible when sharing a semantic model

[ce138867]

When a semantic model is shared in read only access, the user does not have the ability to to see the details of the dax expression for any measures created in the model. This makes it very difficult to understand how the measure is being used/functioning without seeing the expression. Since users also don't have read only access to the data, understanding how the model works also makes troubleshooting and creating new visuals difficult.