[Desktop] Brave Today should not fetch data until interacted with

[petemill]

Whilst Brave Today requests are completely anonymous, we want to make sure we're not making any requests unnecessarily, especially without user interaction. It would be ideal to only request Brave Today data once the user interacts with the feature, either by scrolling down to view the feed or customizing settings.

Test Plan

Since a request has been removed, we mainly need to check that the feed is still correctly fetched and updated:

• Content is retrieved on a fresh profile
• When modifying publishers list (via toggling off and on), the feed still updates to exclude or include the new publishers.

However, in order to verify that there is no fetch before the feature is interacted with AFAIK we will need a third party app (e.g. Little Snitch) to verify no connection is made on startup. Once the browser is started up, we can use the Brave extension DevTools (via chrome://inspect#extensions) to verify that the feed is not fetched when a NTP is opened, and only fetched when either:

• Scrolling down to open the Brave Today feed (and a loading indicator is shown whilst waiting for the first time)
• Opening the customize dialog and switching to the Brave Today tab

[btlechowski]

Verification passed on

Brave	1.18.70 Chromium: 87.0.4280.101 (Official Build) (64-bit)
Revision	9407c80213cda69c2b7abcb4fa8e3f74488f4956-refs/branch-heads/4280@{#1807}
OS	Windows 7 Service Pack 1 (Build 7601.24544)

Verified test plan from the description
Verified the content is fetched after scrolling down

Verified the feed still updates to exclude or include the new publishers