added support for peer review email custom text

[alinvetian]

Closes: datadryad/dryad-product-roadmap#3734

I also removed user_mailer views footer text since it is now in mailer layout

[ahamelers]

@alinvetian I don't think this should hold this from being merged or anything, but it might be better to make like a 'custom_email_text' table and link each journal to that—I learned with the tenant logos how annoying it could become to have potentially very long content like this on the main table.

Also, @ryscher is there any potential that other member types than journals would want to add custom email text in the future?

[alinvetian]

@ahamelers I know having a separate table might be better, but at the moment we only have this simple customization. If we decide to do more custom emails in future I will update this field too, but there is no point at the moment