feat!: Abusive content reporting (#6977)

This implements milestones 1. and 4. from **Task F. Moderation features: Reporting** (part of [amendment of the workplan](https://codeberg.org/forgejo/sustainability/src/branch/main/2022-12-01-nlnet/2025-02-07-extended-workplan.md#task-f-moderation-features-reporting) for NLnet 2022-12-035):

> 1. A reporting feature is implemented in the database. It ensures that content remains available for review, even if a user deletes it after a report was sent.

> 4. Users can report the most relevant content types (at least: issue comments, repositories, users)

### See also:
- forgejo/discussions#291
- forgejo/discussions#304
- forgejo/design#30

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6977
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: floss4good <floss4good@disroot.org>
Co-committed-by: floss4good <floss4good@disroot.org>

services/forms/report_abuse.go

@@ -0,0 +1,28 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package forms
+
+import (
+	"net/http"
+
+	"forgejo.org/models/moderation"
+	"forgejo.org/modules/web/middleware"
+	"forgejo.org/services/context"
+
+	"code.forgejo.org/go-chi/binding"
+)
+
+// ReportAbuseForm is used to interact with the UI of the form that submits new abuse reports.
+type ReportAbuseForm struct {
+	ContentID     int64
+	ContentType   moderation.ReportedContentType
+	AbuseCategory moderation.AbuseCategoryType `binding:"Required" locale:"moderation.abuse_category"`
+	Remarks       string                       `binding:"Required;MinSize(20);MaxSize(500)" preprocess:"TrimSpace" locale:"moderation.report_remarks"`
+}
+
+// Validate validates the fields of ReportAbuseForm.
+func (f *ReportAbuseForm) Validate(req *http.Request, errs binding.Errors) binding.Errors {
+	ctx := context.GetValidateContext(req)
+	return middleware.Validate(errs, ctx.Data, f, ctx.Locale)
+}