Use checkboxes instead of select list for Module Categories on project edit form for better usability

I have noticed a number of drupal.org module projects have accidentally wiped out their module categories or unintentionally removed a very relevant category by selecting other categories and not doing "control click" to select multiple. For example, the Internationalization module used to show up under the Multilingual category, but was recently updated so that it no longer shows up there: #1722544: Internationalization project is no longer marked with the Multilingual module category

If the module category selection was changed from a select list to checkboxes, it would be much easier for the project maintainers to properly classify their projects and there would be less errors when updating the project. This, in turn, provides a better experience to new Drupal users who are searching for modules on drupal.org as more projects would be properly classified.