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>
Picking closest version from the past would be good for the `wayback` parameter mentioned in #1990.

Here, the user asked for a concrete `recid` and `revision` values, so I think we should reply along the lines of  *Record N revision M does not seem to exist", similarly to what we do for records in master, instead of redirecting user to closest or current version.  IOW, we reply strictly, and the client can handle the situation as they want.