[MM-59543] Disallow use of file: protocol in the app, remove all references to it, add mattermost-desktop: protocol to read local files

[devinbinnie]

Summary

This PR is a companion to https://github.com/mattermost/desktop/pull/3094 where most of the security fuses were flipped. However this change flips only one, GrantFileProtocolExtraPrivileges to false and includes a number of changes to support that fuse being flipped:

• I have removed all use of the file: protocol in the app, replacing it with a custom protocol called mattermost-desktop as per the recommendation by Electron. This protocol has a handler that allows only files from the renderer folder to be served up and prevents any path traversal using said protocol.
• As a result of the above I have also removed the method getLocalURLString since it's no longer required anywhere
• One of the files in the loading screen was being loaded directly as an image, so I've amended that and moved that file (StippleMask.jpg)
• The downloads dropdown was directly loading files from the disk for thumbnails, so I've replaced that with the nativeImage library that creates a base64 based thumbnail and sends that along with the other download data.

This change is being separated due to how much has changed to support this, so it should likely be given more time to bake than the other fuses flips.

Ticket Link

https://mattermost.atlassian.net/browse/MM-59543

Disallow use of file: protocol in the app

[devinbinnie]

1. Does this have any effect on what types of files can be linked to from the web app or a web app plugin? I'm thinking of if someone had a custom plugin that opened a file on a network drive or something, but I don't think we supported that anyway.

None of these should affect any of the remote code (ie. webapp code), this is only concerned with loading files internal to the Desktop App. Nothing loaded into the Mattermost web app itself should be affected.

2. Similarly, does this mean that the web app could open mattermost-desktop links? I'm not sure if these changes affect the web app or just the outer Electron processes

This would technically allow any renderer process to use this protocol, however the navigation is blocked via other means, so it will try to open mattermost-desktop: links externally, which of course won't work. This protocol is only registered internally to the Desktop App