r-httpd-browse: add tighter/controlled cluster navigation links

r-httpd-browse: add tighter/controlled cluster navigation links

- cluster linkage from the testrun metadata viewer now passes the
  destination cluster via keyvalue type parameters instead of
  enumerating (potentially very many) commitishes

- added a backup limiting facility to hide cluster-relative diff/regress
  links that are likely to fail anyway

- also in the name of user not accidentally getting too much data,
  dropped the "all testruns" link from top /testruns/ listing

- added a --has-keyvalue to r-diff-testruns, to identify cluster
  to compare to given committishes

- generalized --has-keyvalue[-like,-glob] to --has-keyvalue K V
  [--has-keyvalue-op OPERATOR] for r-find-testruns, with = being
  preferable for cluster direct naviation links from the web frontend,
  so metacharacter quoting doesnt interfere (as it could with %/* in
  metadata value); ditched the previous -like and -glob option, which is
  a cli break; could bring it back if desirable