FIX: update theme fields when updating from ThemesInstallTask (PR #10143)

Theme values were not actually being updated via the theme install rake task, because update_from_remote does not save the theme.

In the admin controller, when updating a theme from remote, we see

And then theme is saved later in the action

The update_from_remote method sets fields on the theme, but does not save the theme.

In the rake task, I was assuming that the theme was automatically saved by update_from_remote', so adding save here is a fix! It is a bit of an interesting paradigm to wait to save theme` outside the update method, but it makes more sense in the context of how the controller uses the method.


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Could we have a test for this?

@eviltrout Test added :+1: