Changed BlogProfileImage publish from onBeforeWrite to $owns

[purplespider]

Redo of #749

Description

Due to the order in onBeforeWrite, the BlogProfileImage added in BlogMemberExtension doesn't get automatically published unless you have also changed the member's name (unlikely).

This is because if FirstName or Surname are unchanged, then onBeforeWrite returns, before being able to publish the profile image.

This PR sets it to use $owns to take care of publishing the image instead.

Manual testing steps

1. Create a new user in Security.
2. Click Create
3. Upload a new "Blog profile image" for the user.
4. Click Save

What I expect to happen: The image gets published.
What actually happens: The image stays as "Draft".

Issues

• Member BlogProfileImage doesn't get published unless FirstName/Surname is changed #748

Pull request checklist

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• This change is covered with tests (or tests aren't necessary for this change)
• Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• CI is green

[GuySartorelli]

Great! Thanks for doing this. Just one tiny change.

[michalkleiner]

I've updated the commit message to have the type prefix as well (it's a FIX) and for it to be a bit more descriptive.