Improved single author deprecated notices

- single authors were deprecated in v1.22 when we added multiple authors
- we always thought we'd clean this up a lot sooner, but it's stuck because it's an annoying thing to break people's shit over
- still saying "remove in vX" isn't useful, we need to know how long a feature has been deprecated so we can judge whether it's safe to remove

core/frontend/helpers/author.js

@@ -14,7 +14,7 @@ const buildInHelpers = hbs.handlebars.helpers;
 const _ = require('lodash');
 
 /**
- * @deprecated: will be removed in Ghost 3.0
+ * @deprecated: single authors was superceded by multiple authors in Ghost 1.22.0
  */
 module.exports = function author(options) {
     if (options.fn) {

core/frontend/helpers/prev_post.js

@@ -18,7 +18,7 @@ const buildApiOptions = function buildApiOptions(options, post) {
 
     const apiOptions = {
         /**
-         * @deprecated: `author`, will be removed in Ghost 3.0
+         * @deprecated: single authors was superceded by multiple authors in Ghost 1.22.0
          */
         include: 'author,authors,tags',
         order: 'published_at ' + order,

core/server/data/schema/schema.js

@@ -38,7 +38,7 @@ module.exports = {
             validations: {isIn: [['none', 'all', 'free', 'paid']]}
         },
         /**
-         * @deprecated: `author_id`, might be removed in Ghost 3.0
+         * @deprecated: single authors was superceded by multiple authors in Ghost 1.22.0
          * If we keep it, then only, because you can easier query post.author_id than posts_authors[*].sort_order.
          */
         author_id: {type: 'string', maxlength: 24, nullable: false},

core/server/models/relations/authors.js

@@ -108,7 +108,7 @@ module.exports.extendModel = function extendModel(Post, Posts, ghostBookshelf) {
             const ops = [];
 
             /**
-             * @deprecated: `author`, is unused in Ghost 3.0, should be removed before Ghost 4.0
+             * @deprecated: single authors was superceded by multiple authors in Ghost 1.22.0 - `author`, is unused in Ghost 3.0
              */
             model.unset('author');
 
@@ -191,7 +191,7 @@ module.exports.extendModel = function extendModel(Post, Posts, ghostBookshelf) {
 
             /**
              * CASE: `author` was requested, `posts.authors` must exist
-             * @deprecated: `author`, will be removed in Ghost 3.0
+             * @deprecated: single authors was superceded by multiple authors in Ghost 1.22.0
              */
             if (this._originalOptions.withRelated && this._originalOptions.withRelated && this._originalOptions.withRelated.indexOf('author') !== -1) {
                 if (!authors.models.length) {