Guard "Link another address" link when SOCIALACCOUNT_ONLY=True

allauth 65+ omits the account_email URL entirely when SOCIALACCOUNT_ONLY=True, not just account_signup (already guarded in base.html, see 1b4f2913).

fragments/send_as.html links to account_email unconditionally in the "Link another address" prompt shown on the reply/compose form whenever a logged-in user has more than one address to send as. That fragment is included from message.html and message_new.html, so opening a message in the archive while logged in with multiple linked addresses raises NoReverseMatch.

Wrap the link in {% if not SOCIALACCOUNT_ONLY %}, same pattern as the navbar guards.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com