Fix missing PWA icons

[umglurf]

When starting up listlings on an new and empty redis, the icon_small and icon_large settings are not set. This causes the icons field i manifest.webmanifest to be empty, and thus the application does not work as an PWA.
Is it supposed to be empty or is that a bug? I was not able to figure out how to set it the correct way, so ended up editing the Settings key in redis directly.
Tested on both 0.18.1 and 0.18.2

[noyainrain]

Thanks for the feedback @umglurf, good point!

First, to set a custom icon, staff members can use "Edit site settings" from the main menu. The first user browsing to a fresh instance of Open Listling automatically gets staff privileges. Thinking about it, this is not mentioned anywhere yet, so we should probably put that in the README. 😅

Is it supposed to be empty or is that a bug?

Originally, before implementing PWA functionality, icons were completely optional and thus empty on purpose. However, because like you mention they are required for PWAs, this is now indeed a bug.

Going forward, I think we should ship a default icon, which can still be overridden by the administrator if they want to have a custom one.

In the meantime, you can set the following values via "Edit site settings" as mentioned above:

• Icon small: /static/images/icon-small.png?v=6547bc98233711f90d23cdde251c780d
• Icon large: /static/images/icon-large.png?v=00cadbcb574ddc20acef8561efdc8d95

If there are still any open questions, please shoot! 😊 Note that I'm on vacation next week and will reply only after I'm back.

[noyainrain]

Depends on #58.