There's currently a race condition in the following spec:
65be7a7880/spec/system/admin_about_config_area_spec.rb (L70-L95)
where the form can be saved before the image uploader field has finished uploading the selected image and causing the assertion at line 94 to fail with the following error:
```
Failure/Error: expect(SiteSetting.about_banner_image.sha1).to eq(Upload.generate_digest(image_file))
NoMethodError:
undefined method `sha1' for nil
[Screenshot Image]: /__w/discourse/discourse/tmp/capybara/failures_r_spec_example_groups_admin_about_config_area_page_the_general_settings_card_can_saves_its_fields_to_their_corresponding_site_settings_312.png
~~~~~~~ JS LOGS ~~~~~~~
http://localhost:31338/assets/vendor.js 15902:14 "WARNING: uppy needs a unique id, pass one in to the component implementing this mixin"
~~~~~ END JS LOGS ~~~~~
./spec/system/admin_about_config_area_spec.rb:94:in `block (3 levels) in <main>'
./spec/rails_helper.rb:552:in `block (3 levels) in <top (required)>'
./spec/rails_helper.rb:552:in `block (2 levels) in <top (required)>'
./spec/rails_helper.rb:513:in `block (3 levels) in <top (required)>'
./spec/rails_helper.rb:503:in `block (2 levels) in <top (required)>'
./spec/rails_helper.rb:460:in `block (2 levels) in <top (required)>'
./vendor/bundle/ruby/3.3.0/gems/webmock-3.23.1/lib/webmock/rspec.rb:39:in `block (2 levels) in <top (required)>'
```
This PR fixes the problem by making the system test wait for the image to finish uploading (with 10 seconds timeout) before carrying out the rest of the system test.
This commit continues work laid out by ffec8163b0 for the admin config page for the /about page. The last commit set up the user interface, and this one sets up all the wiring needed to make the input fields and save buttons actually work.
Internal topic: t/128544.
