invites: emails -> messages, log when sendTo attempted when disabled

A user's lengthy debugging resulted in them figuring out "Invite
emails" being disabled stopped the "/inv" command from sending invites
on discord, which makes sense except the confusing setting name (now
renamed "Messages" in the UI), and the fact that no error was reported.
This setting being disabled is now logged to the console when it's
attempted through the admin page or discord. For #378.

config/config-base.yaml

@@ -1138,7 +1138,7 @@ sections:
     type: note
     depends_true: link_reset
     required: false
-    description: Set the "External jfa-go URL" in General so that links to jfa-go can be made.
+    description: Set the "External jfa-go URL" value in General so that links to jfa-go can be made.
   - setting: language
     name: Default reset link language
     requires_restart: true
@@ -1167,9 +1167,9 @@ sections:
     description: Subject of password reset emails.
 - section: invite_emails
   meta:
-    name: Invite emails
+    name: Invite Messages
     description: Settings for sending invites directly to users.
-    depends_true: email|method
+    depends_true: messages|enabled
   settings:
   - setting: enabled
     name: Enabled