Send a system DM to every user on a self-hosted instance

Problem

Currently, there is no good way for an instance admin to send a message to the entire instance's user base. There is a discussion that was made https://feedback.fluxer.com/p/1280 to suggest control of a nagbar for self-hosted instances. While this is good for indicating maintenance periods, it's not good for relaying messages to the userbase regarding things that an instance admin may want to inform the userbase about, especially if users are not always active. Currently, there is a content tool named "System DMs" within the admin panel that gives the instance admin the ability to send messages to users by means of the Fluxer System Bot (soon to be relabelled as the Megaphone in the next big update). Admins have the ability to send one DM to multiple users at a time, so long as the user ID's are specified. Therefore, there is inherently the functionality to send one DM to all users within an instance if you specify every user's ID. However, this is cumbersome and not efficient, especially as a userbase grows. Use of wildcard (*) will not send a DM to all users with this tool.

Proposal

Allow use of wildcard (*) so that a system DM can be sent to an entire self-hosted instance's userbase through System DMs within the admin panel. This is a quality of life change to improve efficiency for admins if they need to send a message to all users.

Notes

I've been told by Hampus within the Fluxer Developers Community that "there are some background works that already exist for doing full table scans efficiently that [one] could borrow from; it is also a thing [he] could implement in like 10-15 minutes [if he's not busy]." I do not code, so I'm putting this out there for Hampus or anyone else who has time to make a PR for this.