[MM-60308] Add a set of "Developer Mode" settings that allow the user to turn off systems or force the app to behave a certain way

[devinbinnie]

Summary

This PR creates a Developer Mode, hidden behind an environment variable that when set, will unlock certain options allowing users to fiddle with the behaviour of the Desktop App. This mode is designed to aid developers in debugging performance issues with the Desktop App, where we believe certain systems with the Desktop App may be the culprit.

The settings added here are:

• Browser Mode Only: Completely disables the preload script, stops web app from knowing it's in the Desktop App
  • Should be the best indicator of if something we're doing with the web app is causing it to slow down or retain memory
  • NOTE: Disables notifications, cross-tab navigation, unread/mentions badges, calls widget, breaks resizing on macOS
• Disable Notification Storage: Turns off the maps that hold references to the notifications until they've been clicked
  • Good for debugging if we're just holding onto too many references to unused notifications
• Disable User Activity Monitor: Turns off the interval that checks if the user is AFK or not
  • Good for debugging if that is affecting the user status negatively, or causing other strange behavior
• Disable Context Menu: Turns off the context menu attached to the BrowserViews
  • Just a sanity check around if that library is causing anything odd
• Force Legacy API: Forces the app to revert back to the old messaging API instead of the newer contextBridge API
  • Another sanity check to see if the new API is responsible for holding onto memory
• Force New API: Forces the app to use the contextBridge API and completely disables the legacy one
  • Currently the application runs some listeners for the legacy API side-by-side, so this forces them off entirely.

This mode should not be turned on by anyone who isn't a developer, or was instructed by a Mattermost developer to turn this on. When the feature is on, an indicator will show up on the top bar showing that you are in Developer Mode:

Ticket Link

https://mattermost.atlassian.net/browse/MM-60308

Add `Developer Mode` settings to help debug performance issues

[devinbinnie]

@abhijit-singh

Not sure how to test this, which env variable do I change and how?

The variable is MM_DESKTOP_DEVELOPER_MODE and it should be set to true. On Mac, you can run export MM_DESKTOP_DEVELOPER_MODE=true and then run the build from the command line.

But I'm also not sure if this needs any UX testing. We should just make sure it works as expected on Windows too if we haven't already checked that. Other than that, feel free to remove me as a reviewer and go ahead with this.

The reason I asked for UX was to confirm that the indicator (screenshotted above) looked alright, and that the descriptions of the menu items made sense. If that's all okay, you can approve (or remove yourself) and we'll go from there