Link prefixed Wikidata-tags to wikidata.org

This is a follow-up to #786, in which I've linked the basic wikidata-tags to [wikidata.org](https://wikidata.org)

With this pull-request, also the other wikidata-tags (see [the proposal's wiki-page](https://wiki.openstreetmap.org/wiki/Proposed_features/Wikidata)) get linked.

I decided against a catch-all to discourage misspelled keys. If other prefixes will get popular, one could add them easily in the future.

So tags with the following keys are the only ones that get linked:
* architect:wikidata
* artist:wikidata
* brand:wikidata
* ~~name:etymology:wikidata~~
* ~~old_name:etymology:wikidata~~
* operator:wikidata
* subject:wikidata

The reason that it took longer than expected to implement it, was the following:
The basic tag allows only exactly one value, because both OSM and WikiData follow the rule, that one object in reality translates to exactly one object in the database. That makes it a simple 1:1-mapping.
But with the other tags it's more complicated, because one object can have multiple operators, can be named after more than one thing, can cover multiple subjects, …

The only example I currently found was a [cathedral named "Peter and Paul"](http://www.openstreetmap.org/way/38103012) after the apostles [Paul](https://www.wikidata.org/wiki/Q9200) and [Peter](https://www.wikidata.org/wiki/Q33923), which is perfectly valid.

It took some time until I got a satisfying solution, but finally I decided to allow a semicolon-separated list of wikidata-IDs and display them as multiple links to wikidata which are separated by semicolons:
![arbeitsflche 1_002](https://cloud.githubusercontent.com/assets/3904348/3666654/098e6d90-11f5-11e4-8183-8084b38a12da.png)
![arbeitsflche 1_003](https://cloud.githubusercontent.com/assets/3904348/3666661/1f04c28c-11f5-11e4-92dd-cbfea6a817b8.png)
@tomhughes thanks for your comments. I'm quite a newbie in Ruby, so I didn't realize, that using map would be a better solution. I'll revise my changes according to your recommendations.

Regarding the multiple values: No, it's not left over from etymology. For example for <code>brand:wikidata</code> it makes sense to allow multiple values, because a car-shop may sell more than one brand (see examples in my comment above). But the plain <code>wikidata</code>-tag establishes a 1:1-mapping of wikidata- and OSM-items. There won't be more than one wikidata-item representing the very same entity as one OSM-object.