Issue #3500883: Finalize packager admin page

• Implemented batch fired when clicking on "repackage now".
• Removed utterly inefficient loading of all projects in buildForm().
• Used injection whenever possible.
• Not very proud of having to load two kinds of release entities in releaseBatchProcess() but as we said this is a bit of a conception issue.
• I believe specifying releases this way: 6.x% works... provided you do have such releases in your database.

Please, let's not fight on private vs protected and final keywords 😄

Closes #3500883