🐛 Fixed slugs from exceeding db limit (#9251)

closes #8143

Fixed a potential issue (edge-case), where our generated and validated (in terms of check for existance and add a counter) would return a slug, that will exceed the maximum length of the slug fields (191 chars).

This is mostly possible for the post title, which can be 255 chars long and would generate a slug with the same length. This would prevent the user from actually saving a post.

I tried first to determine the expected length for a slug that already exists, but decided that the **easier** and simplyfied implementation is to always cut a slug to **185 chars** (+ counter). This makes it easier to find duplicates and includes a possible high number of counts (edge-edge-case).

The slug will not be cut down to 185 chars if it's an import.

core/server/models/base/index.js

@@ -712,8 +712,18 @@ ghostBookshelf.Model = ghostBookshelf.Model.extend({
             });
         };
 
+        // the slug may never be longer than the allowed limit of 191 chars, but should also
+        // take the counter into count. We reduce a too long slug to 185 so we're always on the
+        // safe side, also in terms of checking for existing slugs already.
         slug = utils.safeString(base, options);
 
+        if (slug.length > 185) {
+            // CASE: don't cut the slug on import
+            if (!_.has(options, 'importing') || !options.importing) {
+                slug = slug.slice(0, 185);
+            }
+        }
+
         // If it's a user, let's try to cut it down (unless this is a human request)
         if (baseName === 'user' && options && options.shortSlug && slugTryCount === 1 && slug !== 'ghost-owner') {
             longSlug = slug;