Add `constants` module about girder_web_components

[zachmullen]

There's a Vue mixin I've found myself using across multiple projects now that would make use of these constants:

/**
 * This mixin exposes helper methods for components that need to check that the current user has
 * a certain access level on a given resource.
 */
export const accessLevelChecker = {
  methods: {
    hasWriteAccess: resource => resource._accessLevel >= AccessType.WRITE,
    hasAdminAccess: resource => resource._accessLevel >= AccessType.ADMIN,
  },
};

Since it's small, central, and common, I think that mixin should also be a candidate for inclusion in this library.