pkp/pkp-lib#5716 Add new manage emails UI and remove email templates list

[NateWr]

This commit replaces the email templates list with a new Manage Emails UI
based on our new Mailables system. It includes the following changes:

• Refactor relationship between mailables and email templates. All email
  templates now assigned to a mailable.
• Adds settings page to edit and assign mailable templates
• Remove email templates management list panel
• Migrate existing custom templates to new discussion stage mailables
• Migrate disabled email template functionality to new context settings
• Add alternateTo attribute to emailTemplates.xml for extra templates
• Give all email templates a name
• Generate custom template keys from the name
• Updates to several mailable names, descriptions, filter settings, etc.

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[Vitaliy-1]

Looks good! The approach to associate custom templates with a default one looks simpler and cleaner than the previous one, with mailable. I left a couple of minor comments