Stabilize snapshot Stripes platform

The "snapshot" Stripes platform (consisting generally of tip of master versions of Stripes and UI modules) by its very nature is unstable. However, frequent webpack build failures currently disrupts the availability of the snapshot FOLIO platforms - folio-testing and folio-snapshot. In order to stabilize the "snapshot" FOLIO platforms I'm proposing the following.

1. Commit a known-good build configuration (yarn.lock) to folio-platform-complete/snapshot and folio-platform-core/snapshot.

2. Have Jenkins perform builds of these branches at regular intervals by ignoring the committed yarn.lock files. If the build succeeds, Jenkins will commit a new yarn.lock to the snapshot branches.

3. When the FOLIO snapshot platforms are built via folio-ansible, honor the latest yarn.lock file in the snapshot branch.

(Note: Since platform-core is a dependency of platform-complete, the extra step of publishing a new platform-core snapshot package to the NPM CI repository is necessary)