Added user merge

[snipe]

This definitely can use some massaging, better logging, etc, but I have a particular need for it right now.

[what-the-diff]

• Added a new route for merging users
• Added the merge view to confirm user merges
• Updated language files with translations and added some more text in the views to explain what is happening when you try to merge users (this action cannot be undone)
• Add an option into bulk actions dropdown on /users page that allows admins/superadmins to select multiple users at once, then choose "merge" from the dropdown menu, which will take them through a confirmation screen where they can pick one of those selected accounts as their master account before deleting all other ones (and moving assets etc over). This should only ever be used if there was bad data imported or synced somehow - it's not intended for normal use cases!