records: exposing record metadata revisions

* Extends `get_record` with optional `revision` parameter

* Exposes revision for record information (metadata) tab
  (addresses #1990)

* Adds a config variable CFG_RECORD_REVISION_URL_PARAMETER
  with default value of "revision"

* Shows appropriate revision export links

* Shows flash messages for invalid and non-existent revisions

Signed-off-by: Bogdan Kulynych <bogdan.kulynych@gmail.com