mod_invites_page: Fix templates including stray inviter name (thanks Menel) The util.interpolation syntax `{var?placeholder}` appears to have been intended as an if-not, but this syntax will always return either the variable or the placeholder-template.

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summary: Allow server administrators to probe any user
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This module lets server administrators send `<presence type="probe"/>`
to any local user and receive their presence in response, bypassing
roster checks.