Add Option to Reset Settings

[jauyong]

Feature description

As a user, I'd the ability to reset options to their default state to help with troubleshooting, performance and to get back to a stable state.

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Acceptance criteria

1. There should be an option to restore demo content
2. There should be an option to restore the theme settings
3. There should be an option to restore plugin settings
4. There should be a confirmation after clicking the "reset" button that the user needs to confirm.
5. There should be a confirmation that settings were restored
6. This should be added to the settings page
7. The user should be made very aware of what is happening when they click the "reset" button
8. If the demo content/plugin/theme is already in restored state, the restore buttons should be inactive

Design
https://user-images.githubusercontent.com/8026823/109770787-48e6ea80-7bfc-11eb-93fe-b3298fe87fb3.mov

Implementation brief

QA testing instructions

Demo

Changelog entry

[jauyong]

@rodydavis please let me know if you're ok with this getting worked on. Note that AC4 leaves it open to see what else can be added.

[jauyong]

@dawidmlynarz to create a design mock

[dawidmlynarz]

@jauyong I have created a proposal for the section holding all the reset options. 'Maintenance' was something that I thought would be a nice fit for the section name. If you can imagine a better one, let me know and I will update the mockups.

I have tried to keep the consistent design with the Google Fonts and Material Icons updates. Once you reset content, the button goes into an inactive state, since there is nothing more to reset (correct me if I'm wrong).

Reset.settings.mov

Looking forward to your feedback!

[rodydavis]

LGTM