[publication] Fix submission notification text to attribute the submitter

[Cryptoteep]

Summary

Fixes #10852.

When a user enables "notify me of new publication submissions" in
my_preferences and a different user submits a new publication
proposal, the notification email received by the first user was worded
as a first-person confirmation:

This is to confirm the project proposal "..." received on ...
We will notify you once this proposal has been reviewed.

This reads as though the recipient themselves submitted the proposal,
which is confusing/incorrect when the recipient is actually being
notified about someone else's submission.

modules/publication/ajax/FileUpload.php's notify() function already
builds a User value (the submitter's full name) and passes it into the
template data for notifier_publication_submission.tpl, but the
template never used that variable. This mirrors the existing
notifier_publication_edit.tpl, which correctly says "... has been
edited by {$User}."

Changes

• smarty/templates/email/notifier_publication_submission.tpl: reword
  the notification to state who submitted the proposal, using the
  already-available {$User} variable, and drop the "This is to
  confirm ... we will notify you" phrasing that implied the recipient
  was the submitter.
• test/unittests/PublicationSubmissionNotificationTemplateTest.php:
  new unit test that renders the template's placeholders with sample
  data and asserts:
  • the rendered text attributes the submission to the actual
    submitter's name,
  • the misleading "this is to confirm" wording is gone,
  • all placeholders expected by FileUpload.php::notify() (Title,
    Date, User, ProjectName) are still present in the template.

No PHP logic changed — FileUpload.php already assembled and passed
the User value; only the template text was wrong.

Test plan

• Added test/unittests/PublicationSubmissionNotificationTemplateTest.php,
  a pure-PHP unit test (no DB/Smarty bootstrap required) that
  simulates the {$Var} substitution performed by Email::send()
  and asserts on the resulting text.
• Could not execute vendor/bin/phpunit / composer test in the
  sandbox used to prepare this PR (no PHP/composer toolchain
  available in that environment). The test was manually verified by
  reproducing the same substitution logic in a standalone script
  against the real template file, confirming the assertions pass.
  Please run the LORIS unit test suite (test/unittests.sh /
  phpunit --testsuite LorisUnitTests) in CI to confirm.