Extend data model for validation issues

Add the Issues pane

1. Add a class to represent the validation issue
2. Extend the data model for an validation issue to
  (1) add a severity level field (useful for identify save-blocking issues later)
  (2) replace single entity with an array of entities (useful for issues involving multiple entities)
  (3) add a coordinates field for highlighting the location of the issue on the map
  (4) add a fixes field for possible automatic fixes

3. Update existing validation modules to use the new data model

modules/validations/validation_issue.js

@@ -0,0 +1,41 @@
+import _isObject from 'lodash-es/isObject';
+
+
+var ValidationIssueType = Object.freeze({
+    deprecated_tags: 'deprecated_tags',
+    disconnected_highway: 'disconnected_highway',
+    many_deletions: 'many_deletions',
+    missing_tag: 'missing_tag',
+    old_multipolygon: 'old_multipolygon',
+    tag_suggests_area: 'tag_suggests_area',
+    map_rule_issue: 'map_rule_issue',
+});
+
+
+var ValidationIssueSeverity = Object.freeze({
+    warning: 'warning',
+    error: 'error',
+});
+
+
+export { ValidationIssueType, ValidationIssueSeverity };
+
+
+export function validationIssue(attrs) {
+    if (!_isObject(attrs)) throw new Error('Input attrs is not an object');
+    if (!attrs.type || !ValidationIssueType.hasOwnProperty(attrs.type)) {
+        throw new Error('Invalid attrs.type: ' + attrs.type);
+    }
+    if (!attrs.severity || !ValidationIssueSeverity.hasOwnProperty(attrs.severity)) {
+        throw new Error('Invalid attrs.severity: ' + attr.severity);
+    }
+    if (!attrs.message) throw new Error('attrs.message is empty');
+
+    this.type = attrs.type;
+    this.severity = attrs.severity;
+    this.message = attrs.message;
+    this.tooltip = attrs.tooltip;
+    this.entities = attrs.entities;  // expect an array of entities
+    this.coordinates = attrs.coordinates;  // expect an array of [lon, lat]
+    this.fixes = attrs.fixes;  // expect an array of functions for possible fixes
+}