Added a custom permalink slug for podcasts

[bernattorras]

Changes proposed in this Pull Request

• Added a custom permalink slug for podcasts

Testing instructions

• Go to Settings > Permalinks and confirm that the current structure is set to /blog/%postname%
• Open a podcast post from the frontend (following the link from the backend)
• Confirm that its URL is using a different permalink slug (ai/podcast-post-name)
• If you're getting a 404, flush the permalinks in Settings > General

Mentions #174

[bernattorras]

Merging into trunk after getting the ok here: #174 (comment)