🐛 Fixed free tier showing in the tiers-only paywall in posts

refs INC-36

- The tiers-only paywall was incorrectly rendering "Free". Example: "This post is for subscribers of the Free, Silver and Gold tiers only"
- Steps to reproduce the issue:
    1. Create a post with public visibility, publish it
    2. Then swap the visibility to specific tiers. The default selects all paid tiers. Leave it like that
    3. Update the post. The paywall show Free, even though it should be showing only the paid tiers
- This fix filters out the "free" tier when visibility is set to tiers, before updating a Post. It also throws a loud error when using the bulkEdit action

ghost/posts-service/lib/PostsService.js

@@ -130,6 +130,11 @@ class PostsService {
             }
         }
 
+        // If the visibility is set to specific tiers, make sure that only paid tiers can be set
+        if (frame.data.posts[0] && frame.data.posts[0].visibility === 'tiers' && Array.isArray(frame.data.posts[0].tiers)) {
+            frame.data.posts[0].tiers = frame.data.posts[0].tiers.filter(t => t.type === 'paid');
+        }
+
         if (this.isSet('collections') && frame.data.posts[0].collections) {
             const existingCollections = await this.collectionsService.getCollectionsForPost(frame.options.id);
             for (const collection of frame.data.posts[0].collections) {
@@ -266,7 +271,7 @@ class PostsService {
             }
             let tiers = undefined;
             if (data.meta.visibility === 'tiers') {
-                if (!Array.isArray(data.meta.tiers)) {
+                if (!Array.isArray(data.meta.tiers) || data.meta.tiers.some(tier => tier.type === 'free')) {
                     throw new errors.IncorrectUsageError({
                         message: tpl(messages.invalidTiers)
                     });