Schema editor: only 1 microschema can be selected

With Mesh 1.4, when creating a schema with a field of type micronode, the "allowed microschemas" dropdown only allows selecting one entry. Reproduced both locally and on demo.getmesh.io.

Seen on Linux with Brave, Version 0.70.122 Chromium: 78.0.3904.87 (Official Build) unknown (64-bit)

Start by reproducing the issue in the schema editor with a micronode field and inspect the allowed microschemas dropdown. The work is done when that control permits selecting multiple microschemas, matching the expected behavior described in the issue.

Written by the indexing model from the issue text.