WIP: Include "child" assets in inventory email and other views

[marcusmoore]

Here is the set up for the description that follows:

• asset-001, asset-002, asset-003, and asset-004 are checked out directly to the user
• asset-001-01 is checked out to asset-001
• asset-003-01 is checked out to asset-003
• asset-003-02 is checked out to asset-003
• "Show assets assigned to assets" is enabled in settings
  

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As pointed out in #16811, the email that is sent when clicking the "Email list of all Assigned" button does not include "child" assets (assets that are checked out to the assets checked out to the given user):

But they are included in the view for "Printing All Assigned":

Helpfully they are numbered x.x

This PR addresses that by including the child assets in the email along with the x.x numbering:

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In addition, I noticed that child assets were missing when viewing a user's assets (/users/1#asset) or your own asset's (/account/view-assets#assets).

I've add this to your viewing your own assets:
Before:

After:

• And the last thing to do is add it to viewing a user's assets:
  Before:
  

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fixes #16811