Use email composer when assigning a reviewer

[NateWr]

Describe the problem you would like to solve
When a reviewer is assigned, the editor can send them a message. The UI when sending this email could be improved. It would be good to be able to select any template, use it in any language, and make wider use of email template variables.

Describe the solution you'd like
Use the new email composer UI developed in #5717 when adding or replying to a discussion.

Who is asking for this feature?
Feedback from our UX research, partners and the TC community survey has identified improvements to email sending as a top priority.

[Devika008]

I am taking the email workflows designed for editorial decisions in 3.4 forward and scaling them throughout OJS. You can find the designs for using the new email workflow when assigning reviewers below.

The different completed workflows for assigning a reviewer can be found here: #3022