Allow managing of .gitignore file

[isc-pbarton]

Add options to Source Control menu and Workspace view to "ignore this file from source control". This will add the file to the .gitignore. Make sure this plays well with deployment through the pull event handlers and Import All options.

[isc-tleavitt]

@isc-pbarton I dropped this from the Q3/Q4 milestone - this probably isn't reasonably in scope for end of year. If you disagree we can readd.