episode: localize the episode date string

Date strings would output month names always in English.

Now the chrono unstable-locales feature is used to localize the date string. Redundant formatting code was moved into utils.

Closes #251

Waiting for feedback on: #251 (comment 1382447) then I'll remove the draft status.

In a second commit I also moved the duration formatting code to utils and fixed some panic crashes when the episode duration is above 24h.