Allows distinction between submitted and accepted publications

[djperrefort]

Updates the Publication model with the following changes:

• Publication & Submission Dates
  • Renamed date field to published and made the field nullable.
  • Added a submitted field to track submission dates.
• Journal Metadata
  • Updated journal field to be nullable and limited to 100 characters.
  • Introduced volume and issue fields to store publication details.
• Preparation Status
  • Added the preparation boolean field to indicate if a publication is still in preparation.