Add support for podcast:txt tag

Adding support for the podcast:txt tag in the RSS feed is the last step to Castopod becoming compliant with the requirements for Podcast Standards Project (PSP) certification.

Support should consist of a dedicated field in the podcast admin, with a purpose field not exceeding 128 characters, and a freeform text field not exceeding 4000 characters.

https://podstandards.org/ https://podcasting2.org/podcast-namespace/tags/txt