fix: Include uncategorized articles in the all article section to allow edit/delete (#10153)

Fixes https://github.com/chatwoot/chatwoot/issues/9935
Fixes https://github.com/chatwoot/chatwoot/issues/8213

The articles were grouped by category, with locale being a derived
attribute from the category. If a category was deleted, the article
wouldn't appear on the dashboard. However, due to a bug, it would show
up in the uncategorized section on the public portal, leaving agents
unable to edit or update the article.

To address this issue, I've added a locale attribute directly to the
article. This attribute is automatically set from the category or the
portal's default locale if not supplied. The API parameters now use this
attribute to filter articles. As a result, the dashboard will display
articles even if they're not associated with a category, improving the
overall workflow.

**Main updates:** 
- Add locale attribute to the Article model. Add db migration to back
fill the data based on the above logic.
- Add a new scope search_by_locale and use it instead of
search_by_category_locale.
- Update the ERB template to include the locale filter.
- Move from `joins` to `left_outer_joins` to include the articles with
no categories.

---------

Co-authored-by: Sojan <sojan@pepalo.com>

Gemfile.lock

@@ -819,7 +819,7 @@ GEM
       rack-proxy (>= 0.6.1)
       railties (>= 5.2)
       semantic_range (>= 2.3.0)
-    webrick (1.8.1)
+    webrick (1.8.2)
     websocket-driver (0.7.6)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.5)