Add plugin to redirect users to new profile plugin

[wchargin]

Summary:
We add a new profile_redirect plugin, called “Profile (moved)” in the
UI, which teaches users how to install the dynamic plugin:

The instructions currently pull in Test PyPI sources; we’ll swap these
out for real PyPI once the package is published.

Test Plan:
Create a new virtualenv, install tf-nightly, uninstall tb-nightly,
and run //tensorboard/pip_package:extract_pip_package and install the
resulting wheel. Launch TensorBoard and note that the “Profile (moved)”
plugin appears in the inactive plugins list. Click the “Copy to
clipboard” button and execute the copied pip install command from the
test virtualenv. Relaunch TensorBoard, and note that the dynamic profile
plugin now appears and the “Profile (moved)” plugin is gone entirely.

Unit tests included for the conditional loading behavior. Changing the
attempted import (in ProfileRedirectPluginLoader.load) to a module
that always can be imported (like sys) causes one of the tests to
fail; changing it to one that can never be imported (like zzz) causes
the other to fail.

wchargin-branch: profile-redirect

[wchargin]

Reviewers: @qiuminxu for anything profile-specific (wording, etc.);
@stephanwlee for general frontend.

[stephanwlee]

Besides the rel="import" on the dashboard-style, other comments are inquiries or nits.

[stephanwlee]

Omit this unless you are making use of it. Else, I believe you need to import https://github.com/tensorflow/tensorboard/blob/master/tensorboard/components/tf_dashboard_common/dashboard-style.html in rel="import".

[wchargin]

Yes, thanks! (I’d used this originally but decided that I didn’t need
it; removed the build dep and import, but forgot to remove this include.
Done.)

[stephanwlee]

FYI: since we are only "officially supporting" evergreen browsers, you may use async/await and make this code read better (wait... you are already using async/await :)).

async _copyInstallCommand() {
    const tryCopy = async () => {
      const textarea = this.$.commandTextarea;
      textarea.select();
      try {
        await navigator.clipboard.writeText(textarea.textContent);
      } catch (error) {
        // Fallback approach.
        if (!document.execCommand('copy')) {
          return Promise.reject();
        }
      }
    };

    try {
      await tryCopy();
      this.$.copiedMessage.innerText = 'Copied.';
    }  catch (e) {
      this.$.copiedMessage.innerText = 'Failed to copy to clipboard..';
    }
  }

[wchargin]

Ah, that second try structure is definitely nicer. Done; thanks!

[wchargin]

Thanks for the careful review!

[wchargin]

Yes, thanks! (I’d used this originally but decided that I didn’t need
it; removed the build dep and import, but forgot to remove this include.
Done.)

[wchargin]

Ah, that second try structure is definitely nicer. Done; thanks!

[stephanwlee]

readonly -> readOnly

[wchargin]

Good catch; thanks! Done.

[qiuminxu]

Tested works great, thanks William for the friendlier solution!

[wchargin]

Thanks for the review, @qiuminxu, and thanks for testing!