Support using a macro to initiate a Mod Mail

[baseballlover723]

We have a macro for telling people they are shadowbanned, but we can't really use it very well since there's currently only support for mod mail replies.

This ticket would be to add the Mod Mail drop down to the create Mod Mail screen, and to also support a subject parameter to prefill when being used to initiate a Mod Mail. As well as to add a macros drop down to the Mod Action module's Send Mod Mail tab.

From looking at the code, it doesn't seem terribly difficult to add support for this, since there isn't too much that is novel about it. Hopefully I'll find some time to work on something in the next few weekends.