When editing a Data Template that has dependant graphs, some attributes should not be modifiable

[cigamit]

Describe the bug
When a Data Template has Graphs created, you should not be able to change some attributes of the Data Template otherwise it will break existing graphs.

To Reproduce
Steps to reproduce the behavior:

1. For an existing Data Template, create a Graph using that Template
2. Edit the Data Template, you will notice that the Add new Data Source to the Template is enabled

Expected behavior
You should not be able to modify certain aspects of a Data Template once Data Sources have been created using it.