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

https://github.com/discourse/discourse/blob/fe284ffd06f759a7109601609d71357599c04c0a/app/controllers/admin/themes_controller.rb#L207

And then theme is saved later in the action https://github.com/discourse/discourse/blob/fe284ffd06f759a7109601609d71357599c04c0a/app/controllers/admin/themes_controller.rb#L211

The update_from_remote method sets fields on the theme, but does not save the theme. https://github.com/discourse/discourse/blob/fe284ffd06f759a7109601609d71357599c04c0a/app/models/remote_theme.rb#L176

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.

GitHub

This pull request has been mentioned on Discourse Meta. There might be relevant details there:

https://meta.discourse.org/t/theme-component-upgrade-via-rake-not-working/152322/8

Could we have a test for this?

@eviltrout Test added :+1: