Have plugin submission form validation match Plugins bot validation

To improve the workflow for contributing plugins, all the issues that the automated plugin checker tool will block on should be set as required when submitting the plugin.

As a minimum, I would expect this to include the fields description, bug tracker URL, and illustrative screenshot, (which the bot reported as "Recommended").

I submitted https://moodle.org/plugins/edit.php?plugin=assignsubmission_wordcount, and the bot later told me "Needs more work", with:

• Provide the plugin description
• Provide bug tracker URL
• Provide an illustrative screenshot
• Provide the external documentation URL
• Check the repository name

The plugin check described these as "Recommended" and "Suggested", but as one or both of them prevent release of the plugin, they should be marked as "Required".