Logged in CMS user without permissions may not see email link to file submissions

[emteknetnz]

Follow on from #1225 (comment)

If a content author testing is testing a userform that includes a file upload field and that user does not have access to the folder where the submissions are stored, though they are an email recipient then when they do a test submission they'll get a version of the email that does not include a link to the file submission

Root cause here is (I think, haven't actually confirmed) that SubmittedFileField::getFormattedValue() is used to generate a string for both:

• The email message
• Some text that shows in the CMS to logged in user when viewing a submission

We should update things so that this method is only used within the CMS, and create a new method specifically for the email that always includes a link to the file submission