Filter function

[species]

This PR implements a 'tag filter'.

It creates a new view "byAttribute" in the database and runs queries against it.

You can filter by calling the /place/ endpoint with anything that is not an UUID:

• /place/key returns all objects that have an attribute named "key"
• place/key=value returns all object that have the tag key=value set

[species]

As this is a blocker for the release of the editor since 27 days, I ask again for someone to look over it.

@almereyda, @gandhiano please have a look.

If there is no objection, I will merge it in 7 days (Tu Jan 17).

[almereyda]

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