Enable intercom support

We want to start using Intercom for support. It will need to be opt-in.

Here's the user flow I envisage:

Add a new "Enable Support" button next to the page title.

When clicks it opens a modal explaining how we use Intercom for support and showing the server info we are planning on sending, there is a no thanks button which just closes the modal. Clicking accept then set's an option `hmbkp_enable_intercom` and reloads the page.

Once the option is set instead of an enable support button the user now sees a button saying "Support" clicking that opens the standard Intercom popup.

- [x] "Enable Support" button
- [x] Modal explaining support with decline and accept buttons
- [x] Switch "Enable Support" to "Support" when opted in
- [x] Deactivating and uninstalling should remove the opt-in setting