Brave Today: optimize fetching

[petemill]

• Do not fetch any data from remote until user has interacted with brave today once Fix [Desktop] Brave Today should not fetch data until interacted with brave-browser#13059
• Fix double-fetching due to feed fetch also fetching sources which notifies feed there is an update which then performs feed fetch... Fix [Desktop] Brave Today is fetching feed.json twice in a row brave-browser#13058
• Debounce fetch feed update when user is toggling publishers

Submitter Checklist:

• There is a ticket for my issue.
• Used Github auto-closing keywords in the commit message.
• Wrote a good PR/commit description
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed).
• Requested a security/privacy review as needed.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[bsclifton]

s/doens't/doesn't

[bsclifton]

Tested changes, works good! Code changes look good too. Async startup/shutdown when Today enabled/disabled, debounced re-fetch for publisher pref changes. Very nice 😄

[ryanml]

Works great!

[petemill]

CI passed apart from Windows but that was "unstable" not "fail" so 👍

[petemill]

Pushed changes that were only comment updates

[kjozwiak]

Verification PASSED on Win 10 x64 using the following build:

Brave | 1.20.17 Chromium: 87.0.4280.88 (Official Build) nightly (64-bit)
-- | --
Revision | 89e2380a3e36c3464b5dd1302349b1382549290d-refs/branch-heads/4280@{#1761}
OS | Windows 10 OS Version 2009 (Build 19042.630)

Followed the cases/summary motioned via both brave/brave-browser#13059 & brave/brave-browser#13058 and ran through the following cases:

• ensured that both https://brave-today-cdn.brave.com & https://pcdn.brave.com are not being contacted when:
• launching Brave for the first time
• opening a NTP without interacting/scrolling through Brave Today
• ensured that both https://brave-today-cdn.brave.com & https://pcdn.brave.com are only initially contacted when:
  • user starts scrolling through Brave Today via a NTP
  • open the NTP Customize modal and clicking on Brave Today (will load sources.json from https://brave-today-cdn.brave.com
  • https://brave-today-cdn.brave.com --> downloads feed.json & sources.json
  • https://pcdn.brave.com --> pulls down the images
• ensured that when a change is made/publisher is added/removed, Brave contacts https://brave-today-cdn.brave.com and pulls a new feed.json & sources.json
• ensured that scrolling through the Brave Today feed, only https://pcdn.brave.com is contacted while pulling down images

Scrolling through Brave Today for the first time:

Example 1	Example 2

Scrolling through Brave Today feed:

Example 1	Example 2

Open Brave Today section via Customize for the first time: