#1236 Allow non-linking External IDs

[Ahasuerus]

Allow non-linking External IDs. They will be used to enter IDs of secondary verifications that are not available online, e.g. Reginald-1, Reginald-3 or the Bleiler references which use IDs. It will also allow us to support External IDs which are no longer available online.

[Ahasuerus]

Part 1: The software was updated to allow non-linking external ID types, i.e. ID types without an associated external Web site. The change was made in common/library.py and installed in SVN 287 on 2018-12-31. Keeping the FR open since we still need to decide what new non-linking External ID Types we want to add.

[Ahasuerus]

Part 2 - Added the 5 non-linking External ID Types that we have agreed on. Implemented in scripts/add_non-linking_external_id_types.sql, installed in SVN 291 on 2019-01-01. Closing the FR.