Split source_management.go along its CRUD seams (closes #274)

Pure code movement. Every declaration of
internal/handlers/source_management.go moves unchanged into one of:
webhook_list.go, webhook_create.go, webhook_detail.go, webhook_edit.go
and webhook_delete.go for the webhook pages; event_log.go for the event
log; entrypoint.go for the entrypoint handlers; target_create.go and
target.go for the target handlers; and shared.go for the helpers
several of them use. Only each file's package line and imports are new.

Model: opus-5-5

internal/handlers/target.go

@@ -0,0 +1,59 @@
+package handlers
+
+import (
+	"net/http"
+
+	"sneak.berlin/go/webhooker/internal/database"
+)
+
+// HandleTargetDelete handles deleting a target. A deleted
+// database target's archive writer is evicted and its handle
+// closed; the archive file is left on disk.
+func (h *Handlers) HandleTargetDelete() http.HandlerFunc {
+	return h.deleteChildResource(
+		"targetID", &database.Target{},
+		"failed to delete target",
+		h.evictTargetArchiveWriter,
+		targetDeleted,
+	)
+}
+
+// evictTargetArchiveWriter is evictArchiveWriter for one deleted
+// target, and leaves its archive file on disk for the same reason.
+// A target that is not a database target has no writer, and
+// evicting it does nothing.
+func (h *Handlers) evictTargetArchiveWriter(targetID string) {
+	if h.archives == nil {
+		return
+	}
+
+	h.archives.EvictTarget(targetID)
+}
+
+// HandleTargetToggle handles toggling a target's active state.
+func (h *Handlers) HandleTargetToggle() http.HandlerFunc {
+	return h.toggleChildResource(
+		"targetID",
+		func(webhookID, childID string) (bool, error) {
+			var tgt database.Target
+
+			err := h.db.DB().Where(
+				"id = ? AND webhook_id = ?",
+				childID, webhookID,
+			).First(&tgt).Error
+			if err != nil {
+				return false, err
+			}
+
+			// Only the active column: saving the whole row would
+			// write back the name and settings read above over an
+			// edit saved since.
+			active := !tgt.Active
+
+			return active, h.db.DB().Model(&tgt).
+				Update("active", active).Error
+		},
+		"failed to toggle target",
+		targetActivated, targetDeactivated,
+	)
+}