Separate email/letter template for desk rejection VS review stage rejection

[S-Haime]

Currently, there is one email template for rejections (review_decision_decline). It would meet editors' needs better to have two separate templates for desk rejections VS rejections at the review stage.

Having two templates (review_decision_decline + submission_decision_decline perhaps?) means editors can set up two standard responses, limiting how much they may need to edit these templates. Currently, at least one journal has two letters in the current template from which they delete one. This does however introduce more margin for error and issues, if editors need to edit this email every time it is being send.

(Additional note: based on FD ticket 7813. Requested to be put in 1.6 + 'Good first issue' potentially?)