Prevent posting plugin

[thessakranendonk]

PR for issue #312: Warning about bad dock_image tag name does not prevent upload
and the warning shows up only for a brief amount of time before a successful upload sign replaces it.

• Fixed POST request form being sent if form is not populated to satisfaction.
• Fixed error message disappearing after a few seconds
• Fixed wrong error message display (message: Please enter the Docker Image and Public Repo. But repo is correct.)
• Added disabling of submit button until all text input fields are populated
• Changed error message display location from the top of the form to the corresponding field to match other forms UI in ChRIS_ui and ChRIS_store_ui.

Before:

After: