Implement more database methods and basic API routes (#50)

* feat: Implement more database methods & add mod and version routes

* feat: Implement deleting mods/versions & implement categories

* feat: Implement routes for categories, game versions & loaders

* feat: Reorganize API routes in a (hopefully) usable way

migrations/20200812183213_unique-loaders.sql

@@ -0,0 +1,5 @@
+ALTER TABLE game_versions
+ADD UNIQUE(version);
+
+ALTER TABLE loaders
+ADD UNIQUE(loader);