allow larger integer values in the distribution.number_events field

[tiborsimko]

Currently, the maximum number of events a dataset can have, as stored in the distribution.number_events field, is limited to an "integer" value, which is 2147483648.

This is insufficient for the forthcoming ATLAS open data release that will contain a dataset record with 5383448881 events.

We should amend the distribution.number_events field mapping to allow an unsigned integer or a bigint there.

How to reproduce the problem:

$ sed -i 's,25423849,2147483648,g' cernopendata/modules/fixtures/data/records/cms-primary-datasets.json
$ docker exec -i -t opendatacernch-web-1 cernopendata fixtures records --mode insert-or-replace \
    -f  cernopendata/modules/fixtures/data/records/cms-primary-datasets.json
...
opensearchpy.exceptions.RequestError: RequestError(400, 'mapper_parsing_exception', "failed to parse field 
[distribution.number_events] of type [integer] in document with id '025bcb3a-9933-4dc1-9f89-43c2b0cacad0'. 
Preview of field's value: '2147483648'")