Bug 26368: Add OCLC options to marc21_leader.tt

Bug 26368: Add OCLC options to marc21_leader.tt

See documentation at:
https://www.oclc.org/bibformats/en/fixedfield/elvl.html

To test:
 1 - Apply patch, updatedatabase
 2 - Edit a record in the basic editor
 3 - Edit the leader using the plugin
 4 - Note the options for '17 - Encoding level'
 5 - Find the system preference 'UseOCLCEncodingLevels'
 6 - Edit the record again
 7 - Note new options I,J,K,M
 8 - Set the value of encodign level to one of the new options and save
 9 - Edit again
10 - Confirm the value is displayed correctly selected
11 - Repeat tests using advanced cataloging editor

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>