Change analyzer

One approach to solving #73: making `keyword` the default analyzer. This way, new fields are just indexed literally, no tokenisation or turning into lower case, so the normal term match filter can be used.
The nice thing about this approach (compared to #89) is that it is simple and predictable, a search filters for exact matches. Examples:
`search?user=Alice` will *not* return annotations with user value `alice`
`search?myfield=bla` will *not* return annotations with myfield value `bla bla`

I think it still makes sense not to analyze these if we don't intend to search on them. Otherwise, it's just wasted space in the index.