Improved reports flow for self-host instances

Custom Fluxer instances have their own rules and terms of service. The current reporting system doesn't allow these differences to be reflected, which makes the reporting process confusing for users.

Make the entire reporting flow customizable:

• Custom starting options.

Allow instance administrators to define the initial report categories.

• Custom flows for each option.

Each category should have its own reporting flow with different steps and choices.

• Custom report endings.

Allow flows to end without submitting a report if the selected issue doesn't violate any rules, with a custom message explaining why.

• Configurable flags for final steps.

Allow users to submit a plain-text comment. Allow users to attach files to their report. Mark reports as urgent or critical. Support custom flags to help moderators categorize, prioritize, and handle reports more efficiently.