web search: allow to filter by origin type

Due to the high amount of origins we have in the archive, it is at present very challenging to look for, say, a specific python package" (from pypi), or the same for cran, etc., or even all results for listers that don't return a ton of entries.

There are ways around this, but they are pretty awkward. For instance, one can reasonably well find a cran package by adding "r-projet" and "cran" as keywords to search, just because the narrow down decently enough our URL origin pattern for CRAN. But, well, meh.

A quick fix for this would be adding a drop down menu to select origin types to the search UI.

This would not be needed with T2254, but I don't think we're gonna prioritize the search language soon and, given our current focus on adding more listers/loaders, //some// solution is badly needed.

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Migrated from T2869 (view on Phabricator)