add "Open in App" button to website

[memo33]

This adds a button to the website to open a package in the GUI.

The GUI must already be running (on the default port 51515), as the button sends a POST request to the API.

This is a cross-platform solution that avoids registering a custom URI scheme. A custom scheme handler could maybe still be added in the future for Windows and Linux, which would launch the native GUI application.

With this approach, external websites could link to the package page on https://memo33.github.io/sc4pac instead of linking to the GUI directly.

A remaining open question at this point is whether each channel is going to get its own website, or whether packages from other channels would be displayed on the main website, using URLs such as:

https://memo33.github.io/sc4pac/channel/?pkg=jasoncw:elks-building&channel=https://zasco.github.io/sc4pac-channel/channel/

[memo33]

Related to a discussion in memo33/sc4pac/issues/49. @sebamarynissen

[Zasco]

I started working on a browser plugin as mentionned here. I'm waiting for this to complete to implement the same feature. Related to memo33/sc4pac-gui#13.

[Zasco]

I started working on a browser plugin as mentionned here. I'm waiting for this to complete to implement the same feature. Related to memo33/sc4pac-gui#13.

Could you release a snapshot with this feature so I can test it out?

[memo33]

Sure. I'm going to create a proper release then, as the change to the website would depend on it.

[memo33]

I've just released version 0.2.1 of the GUI and 0.5.1 of the CLI. Two remarks:

• As contemplated above, I've added support for the channel URL parameter which allows packages from other channels to be displayed on the website (as a proof-of-concept for now). For example:
  https://memo33.github.io/sc4pac/channel/?pkg=jasoncw:elks-building&channel=https://zasco.github.io/sc4pac-channel/channel/
  Such a link could be used from ST to refer to this package, for example.

• One major issue of the approach taken by this PR is that the POST request to localhost can get blocked by ad blockers. That's why I'm questioning whether this was the right approach. It kind of makes sense that you wouldn't want random websites poking around in your localhost. I've opened "Open in App" button functionality is blocked by browser sc4pac-gui#15 for this.