Create custom IDX template for Mesop

[wwwillchen]

This sets up a custom IDX template for Mesop, following:
https://developers.google.com/idx/guides/custom-templates

How to use:
https://idx.google.com/new?template=https://github.com/wwwillchen/mesop/tree/idx/idx/

I tried this several times and it seems to sometimes work, inexplicably. @richard-to let me know if you're able to run this flow without issues.

Fixes #351.

[richard-to]

It didn't work for me.

Got

 *  Executing task: /nix/store/dq60bgq0l71i5939j9g7v3759h19mdg2-install  

 *  Executing task: /nix/store/dq60bgq0l71i5939j9g7v3759h19mdg2-install  

/nix/store/dq60bgq0l71i5939j9g7v3759h19mdg2-install: line 2: pip: command not found

 *  The terminal process "/nix/store/dq60bgq0l71i5939j9g7v3759h19mdg2-install" terminated with exit code: 127. 
 *  Terminal will be reused by tasks, press any key to close it. 

Then I tried to open a terminal and it did that thing where the terminal does not open.

Also it apparently does not work if third party cookies are disabled.

[wwwillchen]

It didn't work for me.

Got

 *  Executing task: /nix/store/dq60bgq0l71i5939j9g7v3759h19mdg2-install  

 *  Executing task: /nix/store/dq60bgq0l71i5939j9g7v3759h19mdg2-install  

/nix/store/dq60bgq0l71i5939j9g7v3759h19mdg2-install: line 2: pip: command not found

 *  The terminal process "/nix/store/dq60bgq0l71i5939j9g7v3759h19mdg2-install" terminated with exit code: 127. 
 *  Terminal will be reused by tasks, press any key to close it. 

Then I tried to open a terminal and it did that thing where the terminal does not open.

Also it apparently does not work if third party cookies are disabled.

I just tried this twice and it worked the first time, and then the second time it failed, with the same error as you. I'll file a bug with IDX but I don't think this is usable at the moment.