DAP Publishing: fix DAP API issues

[RayPlante]

This PR addresses the API problems enumerated in OARDEV Publishing issue #39 ("DAP tool DBIO issues"), which point errors that occur when trying to edit or delete certain parts of a NERDm record. A common problem affecting several of the items was that an incorrect version of ejsonschema was being installed in oar-docker's dbio container (see oar-docker PR# 177). Beyond that, this PR makes the following changes:

• the code for creating (on-the-fly) more lenient schemas for checking the result of update requests was replaced with an implementation that will be much simpler to expand. Furthermore, this new implementation removes documentation tags from the lenient schemas which should make validation error messages more compact. Much of this implementation is pulled in from the oar-metadata submodule (see oar-metadata PR# 61).
• support for clearing parts of a NERDm record is added/fixed in the web interface

To test, check out the test/dbio-missing-api branch of oar-docker, build and run it, and then test that the DAP can properly update data mentioned in OARDEV Publishing Issue #39, including the home page and references.

[RayPlante]

Tested by @chuanlin2018