Added extensions panel to about:preferences

[bradleyrichter]

Test plan

See #8065

Original issue description

To increase awareness of Brave's extension support, and to funnel new extension requests, a new prefs panel will be created.

features:

• all brave extensions will appear in a sortable list, sorted alphabetically by default
• each extension can be enabled/disabled with a switch at the right side
  (this should toggle the presence of the extension in the point of integration)
• if an extension is available but not installed by default, the switch begins as an "install" button
• a rt-click menu is available to uninstall any extension that was once manually installed
• a notification bar is triggered when new extensions become available to alert the user of the good news (this alert may be disabled)
• a link is positioned at the bottom of the page to invite users to submit extension requests through the community portal

notes:

• this panel is for extensions only and will not display plugins such as Flash or Widevine
• after implementation, we will add a menu item to point users to this panel
  (Window>Extensions Manager...)

[willy-b]

Any possibility to control other extension permissions in this or future mockups?

In Google Chrome, incognito mode / file URL permissions controls are exposed in their panel:

(you might do it in your popup menu or elsewhere, but the functionality is useful)

[bradleyrichter]

Context menu should work as we are able/need to add this type of functionality. In the future, this will change based on the ability to open the extension API.

[bsclifton]

Pulling into 0.14.1

[bsclifton]

Per discussion with @bbondy, let's

• not show the version if it's not installed (worst case scenario, we we can display "latest")
• not handle the extension updates. I created Add update status to extensions panel #7822 to track this

[NejcZdovc]

We also need to add on this panel enable/disable in the private/session tab, same as Chrome has it "Allow in incognito"

[luixxiul]

How could I reload PDF reader which I accidentally removed?