MT#6195 API: rename rewrite -> rewriterulesets
MT#6195 API: rewriterule/rewriteruleset: POST
MT#6195 API: adapt RewriteRules to search framework

see MT#6283
MT#6195 API: rewriterule/rewriteruleset basic tests
MT#6295 API: fix tests Prefer header no longer required
MT#6195 API: ability to handle rules directly in rewriteruleset
MT#6195 API: rewriterule/rewriteruleset: DELETE

and remove some redundant checks (done by formhandler)