Add an admin option to require all image attachments to have alt text

Requested by this comment:

I would like [the feature to allow alt and title text to be set for images] implemented with an administrative option to set the alt text as required for an installation. Since all uploaded images can be viewed in Confluence, these values should be set as they are uploaded. The editor could then make this entry the default value in the editor and make it available to edit if needed for that particular use of the image.