Convert add-on submission time to seconds

[nvdaes]

Link to issue number:

Part of #16681, fixup of PR 17091

Summary of the issue:

Add-on submissionTime in json metadata files is in milliseconds, but it needs to be converted to seconds to be transformed in a publicationDate

Description of user facing changes

None

Description of development approach

Convert add-on submissionTime to seconds in addonStore/models/addon.py

Testing strategy:

Tested manually: now Windows App Essentials 20240918.0.0 dev is shown properly with its publication date.

Known issues with pull request:

None

Code Review Checklist:

• Documentation:
  • Change log entry
  • User Documentation
  • Developer / Technical Documentation
  • Context sensitive help for GUI changes
• Testing:
  • Unit tests
  • System (end to end) tests
  • Manual testing
• UX of all users considered:
  • Speech
  • Braille
  • Low Vision
  • Different web browsers
  • Localization in other languages / culture than English
• API is compatible with existing add-ons.
• Security precautions taken.

@coderabbitai summary