Add new 'getCustomVacabulary' function in superdesk-api [SDESK-7268]

[devketanpro]

Add new getCustomVacabulary func in superdesk-api which we used in supedesk-analytics.
superdesk/superdesk-analytics#151

[tomaskikutis]

Do we have a clear definition of what exactly is a "custom" vocabulary? I'd rather expose a method to get all vocabularies from here

[devketanpro]

yep, custom vocabularies means which vocab we can add via metadata settings.
In the backend we fetch this type of vocab by query :- "field_type": None, "service": {"$exists": True}

[tomaskikutis]

Since I was looking at vocabulary code I also pushed some improvements.

Regarding your code, one improvement I made is to keep API slimmer is to only expose isCustomVocabulary method. I also exposed a method to get all vocabularies so you can later filter them using isCustomVocabulary.

Let me know if you have other improvements in mind.