Rethink deletes with "Remove from Source Control" option

[isc-pbarton]

The "Remove from Source Control" option in the source control menu will delete the file from the Git repository. There should at least be a confirmation dialog before doing this delete. Potentially this option should not be available in Basic Mode. And perhaps removing an item from source control should not actually delete something from the Git repository, just remove it from tracking in IRIS.

[isc-tleavitt]

Related to #455 - we missed the target on the original fix, though there's still value in what was done there.

[isc-tleavitt]

I think it could also be on the table to remove this menu item entirely. A broader consideration could be how this extension plays alongside .gitignore files (could be edited via the web application, or maybe managed through a UI somewhere eventually?)

[isc-pbarton]

Gitignore management has been rolled into a separate issue: #630