Fix missing thumbnails in mobile search

[jankrcal]

This PR addresses a few minor issues in search:

• fixes missing thumbnails on mobile (a new style introduced in Support podcast episodes in web-core #168 in search.scss was unintentionally applied only to desktop)
• tweaks styling of the overlay text on mobile (making it smaller to better fit the small size of the thumbnail; especially important for the longer "Podcast 2050" overlay)
• makes podcast open in a new window/tab from search (to make the behavior consistent with usages in boxes and elsewhere) - podcasts are detected by the URL being absolute, starting with a URL scheme like "https://"

[jankrcal]

@katkolouchova Could you please take a look?

This is semi-urgent as it fixes a nasty visual bug in mobile search. Anytime tomorrow is okay. If you can't make it, please let me know!

This could be a complexity of a change that you could soon handle independently but feel free to disagree ;)

[katkolouchova]

I've tested it locally and it seems to be working well. The only thing I'm a bit hesitant about is the overlay of "Podcast 2050" and episode number (see the screenshot below). Moving the episode number up would in my opinion look better, but perhaps this is something you've already discussed and agreed on with Petr? However, for the time being, I think it can be merged as it is now.

[jankrcal]

Yeah, that sucks but I am working with Petr on a proper fix.

I don't want to delay this PR further.

[jankrcal]

For the record, the episode number is part of the jpg image so I can't easily move it.
Irina has to create new assets for this use case (without episode numbers).

[katkolouchova]

Yes, I've figured. Good to hear that you are already working on that.