Improve gallery

This may be split into multiple sub issues.
The gallery is today generated from a Json file which is cached contents generated by the Thingiverse API. This has a number of improvement possibilities:

• The cached Json file isn't updated. Set up a cron job or smth. requesting a new file regularly. We might need to move to a different hosting infrastructure for this to work.

The gallery examples are just the N latest openscad files on Thingiverse. We should collect more representative models. Ideas are:

• Create one or more collections on Thingiverse (using the openscad account), and show the contents of those collections in addition to the random selection
• Look for models from other sources than Thingiverse and create (handcode?) a similar structure for displaying those

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Start by tracing how the gallery is generated from the cached JSON file and how that file is obtained from the Thingiverse API. Review the existing gallery structure before deciding which separately scoped improvement to pursue: regular updates, representative Thingiverse collections, or models from other sources; done should be defined by the selected sub-issue.