AlternativeTo: cleanup code for tutorial

//cc @russellholt @jagtalon @jdorweiler 

- Fixed indentation, now all spaces
- Converted private `get_platform()` fn to a HB helper
- Removed `normalize` fn it was only modifying one property, `Platforms` (title isn't being used in the template)
- Removed `sourceIcon`, not necessary
- Removed trailing space from `itemType`
- Added some spacing to the template for legibility
- Removed unused "alternative_to_details.handlebars" template