Custom error messages for file-size instance limits

Problem

For self-hosted instances with upload limits, some communities may offer alternative file sharing/hosting services that are more suitable for large files. The current upload error message is hardcoded into Fluxer, which is less convenient when trying to convey information such as "You can upload large files to <URL>" to community members.

Proposal

Instead of the existing hardcoded error message string defined here: https://github.com/fluxerapp/fluxer/blob/ad99de22bc1dea150b2c43baed0fa010d84852c0/fluxer_app/src/features/messaging/components/alerts/FileSizeTooLargeModal.tsx#L40 A variable that defaults to the above string, configurable within the instance configuration would be the simplest way to implement this.