improve the functioning of keywords and keyword-based navigation

[bansp]

Currently, keywords are (nearly) completely unconstrained. They are listed:

• at https://standards.clarin.eu/sis/views/sanity-check-keywords.xq (with numbers and links to format descriptions)
• on the main (index) page, in the keyword cloud
• in format descriptions, below the recommendation table: see e.g. PDF/A, AIFF

The label for issues regarding keywords is SIS:keywords.

Keywords should most probably be split into those that remain unconstrained -- to assist in identifying new useful subcategories, and those that follow established controlled vocabularies.

• prepare a schema
• add sensible vocabulary
• add more vocabularies, if needed