records: exposing record metadata revisions

(addresses #1990)

* Extends `get_record` with optional `revision` parameter
* Exposes revision for record information (metadata) tab
* Adds a config variable CFG_RECORD_REVISION_URL_PARAMETER
  with default value of "revision"

Signed-off-by: Bogdan Kulynych <bogdan.kulynych@gmail.com>
Please add a FIXME note here that this code block works only for MARC and please reference #1990 that speaks about the task of storing/exposing past recjson versions for any master format.  (That we should tackle later; CC-ing @egabancho.)