Brave News Desktop: Following or unfollowing sources and channels should trigger an update banner

[petemill]

When channels or sources follows are changed in Customize, we should call the redux action checkForUpdate similar to how this happens in the redux setPublisherPref action.

note: That happens in a less than ideal manner since it waits for 3 seconds after making changes, hoping that the backend has re-fetches and re-built the feed in that time. Ideally we should have an event mechanism through a new mojom interface. We don't neccessarily need to improve that for this issue fix.

Steps

• Open Customize
• Follow a new channel
• Close Customize
• Actual: No "reload content" button pill is available
• Expected: A "reload content" button pill is shown

[stephendonner]

Verified PASSED using

Brave	1.48.118 Chromium: 109.0.5414.80 (Official Build) beta (x86_64)
Revision	0f69b168d36a06cace4365e9f029fa987afa5633-refs/branch-heads/5414@{#1178}
OS	macOS Version 11.7.2 (Build 20G1020)

Steps:

1. installed 1.48.118
2. launched Brave
3. opened a new-tab page
4. opted in to Brave News
5. clicked on Customize
6. unfollowed Top News
7. followed Cars
8. waited
9. confirmed I saw Load new content
10. clicked on the button
11. confirmed I saw content from The Drive subbed in

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