Dev/#278 display external stream info

[gammaw]

Main changes:

• Add displaying relayed streams feauture (see Wiki for usage: https://github.com/lahmacunradio/frontend/wiki/How-to-handle-relayed-streams-as-shows);
  • Main logic: so far the player info (title, subtitle, image link) was solely filled based on the nowplaying API answer from the streaming server (Azuracast); this new feature introduces a fallback mechanism if such info is not available. Specifically, in case of stream relays there is no image link and the title and subtitle fields depend on the streamer. We use the combination of the playlist (empty) and is_live (false) fields to detect such a case and we fallback to compute the show based on the current CET time and arcsi.
• Fix some CET issues (CET display and calculating CET times to be in sunch with arcsi schedules);
• Start using store (Vue's "global variable" concept) for sharing/reusing data across components (most notably in this PR: radio player component computes show info such as title, subtitle which is reused by, e.g., schedule on home page).

Fixed issues:

• https://github.com/lahmacunradio/frontend/issues/278
• https://github.com/lahmacunradio/frontend/issues/280

Validation:

• Tested locally with relay streams and for regression (live, scheduled shows etc.),
• Deployed on dev server.

Known issues (to be done later) and disclaimer:

• https://github.com/lahmacunradio/frontend/issues/279
• Commits are sometimes chaotic, I needed to backtrack here and there.
• Internal arcsi API (meant for arcsi UI) used to request unarchived show items too (see in usage doc above).

Please review until Monday if you can and sorry for the very short notice! It got to be a more complicated feature than I expected.

[gammaw]

Thanks @tuz666! As a best effort and last minute reaction, I pushed a minimal change and replied on each of the comment. Please check.

[tuz666]

Thanks for the quick fixes, I will create issues/cards for the remaining stuff.