Renaming or migrating a blog category moves it to the end of the categories of its blog posts

When a blog category is renamed, or migrated from the legacy Blog space to Blog.Categories, the blog posts that are in this category are updated, but the category is moved to the end of the categories list of each post instead of keeping its position.

Steps to reproduce

1. Create a blog post in the categories News, Other and Personal (in this order)
2. Rename the News category (through a page rename / move) to Announcements
3. Edit the blog post

Expected: the categories are Announcements, Other, Personal
Actual: the categories are Other, Personal, Announcements

The order of the categories is user data (it is the order in which they are displayed for the post).

Cause

The old value is removed from the list and the new one is appended, instead of being replaced in place:

• CategoryRenameListener.java:171-172
• CategoryLocationMigration.java:335-336

The current behaviour is pinned by CategoryRenameListenerTest#renamingACategoryUpdatesTheBlogPostsInIt and CategoryLocationMigrationTest#migrateMovesTheLegacyCategoriesToTheCategoriesSpace (BLOG-269).