Use email composer UI when assigning a participant

[NateWr]

Describe the problem you would like to solve
When assigning a user to a submission, the editor can send them a message. The usability of this workflow could be improved. It would be good to be able to select any email 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 assigning a user to a submission.

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 participants below.

1. 

On Clicking "Assign"

2. 

• You can view the participants already assigned to a submission in the same way we can view authors assigned when appointing reviewers. This is done to increase resonance and increase recall factor
• When assigning participants you can either "locate an existing user" or "add new user"

On Clicking "Locate a User"

3. 

• When you locate an existing user you can search for them via their roles, name or usernames. Current UI seemed confusing to a few participants and hence have modified it a bit to show the different ways in which you can search for a user
• a Table will appear detailing user details which can be selected by the JM
• Only one user can be assigned at a time

4. 

On Clicking "Add New User"

5. 

6. 

7.