projector: Fix standalone vz_projector server.

[bmd3k]

See: #6065

We document the ability to run the projector plugin in standalone mode:
https://github.com/tensorflow/tensorboard/blob/master/tensorboard/plugins/projector/README.md

But it doesn't work.

The first thing to fix is to get the ":standalone" target to be executable and bring up a working server:

• Hard to know exactly where things went wrong. I have found evidence that this worked as late as September 2020. I suspect things went wrong in April 2021, when tensorboard_html_binary() was replaced by tb_combine_html().
• In the case of vz_projector it seems we can just delete the current ":standalone" target and rename the ":standalone_lib" target to ":standalone". tb_combine_html() doesn't seem to be necessary for any part of the build. tf_web_library() produces an executable web server (Amazing! Who knew?).

The second thing to fix is to get it to render a working page.

• As is, the page would wail with the error message "Cannot read properties of null (reading 'appendChild')". In this case, the null property is document.body.
• Again, not really sure where things went wrong but this is probably also related to the changes in April 2021.
• The fix here is to load the js binary in the body tag (much like we do for //tensorboard target).

The third thing to fix is to get a constant port of "6006".

• As is, the web server would start up with random ports.
• The relevant change here is test: fix concurrent web test execution #3047, where port is set to '0'. But the reason for that change is no longer relevant - the test infrastructure the change was supporting has all been deleted.
• The fix is to change the port for the web server back to '6006'.

[rileyajones]

You mention in your description that the webserver can start with "random ports" but the user can also specify the port with an environment variable. In the future cleanup it would be nice to have this respect the environment variable.

[bmd3k]

I may have missed that: What is the environment variable that users can specify?

[bmd3k]

Discussed offline: We don't believe that we currently offer ability to set the port.

[nfelt]

tf_web_library() produces an executable web server (Amazing! Who knew?).

Ah yes, a glorious jart-ism from bygone days. Even more amazing, but dutifully removed by William several years ago, was the huge, cryptic ANSI art image the server used to dump to the console on startup.

More seriously, in a post-Polymer world I expect we would want to avoid any further dependency on rules_closure, which would mean giving up the amazing hidden web server. But I suppose if that world somehow still includes a standalone mode for the projector, we could port it to use some other one-off server binary instead.