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/index.js

@@ -1,5 +1,6 @@
 export { validationDeprecatedTag } from './deprecated_tag';
 export { validationDisconnectedHighway } from './disconnected_highway';
+export { ValidationIssueType, ValidationIssueSeverity } from './validation_issue';
 export { validationManyDeletions } from './many_deletions';
 export { validationMapCSSChecks } from './mapcss_checks';
 export { validationMissingTag } from './missing_tag';