Allow clients to preserve full channel histories locally

Problem

As described on the Attachment Expiry help page, attachments are deleted eventually. I appreciate that Fluxer isn't trying to be a persistent, infinitely growing file host, demanding ever-increasing amounts of money to store an endless stream of memes; that's important to the goal of remaining unburdened by corporate sponsors. At the same time, however, persistence is critical functionality for longstanding communities of all sizes. I and many others regularly search back through, and re-read/refer back to, old conversations from many years ago, because they are often relevant to conversations occurring today. If attachments disappear from those old conversations, important context will often be lost: PDFs of relevant studies will no longer be present, graphs and charts will vanish, etc. And also it's just fun to see old memes sometimes, like a time capsule.

Proposed solution

I would like to see two things added to Fluxer, two things that work in tandem:

1. A community- and channel-level setting for administrators, that toggles whether individual clients are allowed to store full message histories in the channels/threads they have access to, including all attachments.
2. Client-level settings for users, for handling:
   • Whether they wish to store histories locally at all
   • Which communities/channels they wish to do this for
   • How storage should be restricted, and to what extent (how much file space should be consumed, at most? how long into the past should stored histories stretch?)

Ideally, stored histories should be easily shareable and viewable offline. In this way, longstanding communities can seamlessly take responsibility for their own long-term data retention, and share full histories with newcomers if desired (so that they too can access the entirety of the community's past without missing potentially important information), without imposing any additional load on any instance of Fluxer (particularly the "official" one). This improvement would also make general preservation efforts trivial -- Discord has no good way of saving the contents of communities, and in order to accomplish this currently it is necessary to use a Discord bot in conjunction with something like the Discord Chat Exporter.

Notes (optional)

Adding some remarks from a close friend who's not had the time to reply directly: 1a. The app absolutely must support being able to specify an external source to pull the attachments from such as a different server, in addition to just the hard drive. 1b. In support of 1a, file validation is required, not optional. 2a. Being able to merge differences. If Backup A has months 1, 3, and 5, and Backup B has months 2, 4, and 6, the app must be able to combine them into Backup C that has 1-6 without redundancy. 2b. In support of 2a, and for easier sharing, the internal structure of the backup must be sorted on a per-month level -- directories for each year, then each month in that year, being necessary. I do not just want a blob of all the files in one place.