Metadata source "Entity ID" validation can be circumvented

While testing another story, it was noticed that Selenium could attempt to create a metadata source with an id that already existed and could make it to the end of the form before getting a nasty error (409 / undefined toast)

This is reproducible in two ways:

1. Add a new metadata source from the dashboard before the dashboard finishes loading the existing metadata sources
2. Click through to add a new metadata source and refresh the page on the name / entity id section of the form.