Add the switcher to change versions and locales.

[xxr3376]

Create a simple version switcher to change versions and locales.

Features

• User can enable version switch feature by setting theme_use_version_switch=True and set theme_version_switch_json_url = "/versions.json", then you will get an nicely dropdown menu that render everything from /versions.json
• User can enable locale dropdown as well by enable theme_version_switch_enable_locale
  • different locales may have different versions (I believe this will happen frequently due to translation progress)
  • If the current version doesn't exist in the target locale, it will redirect to the default version in the target locale

TODO

• Fix page suffix do not match requirements error when pagename is index.html

Questions

• Can we use ES6 features? Move all javascript into index.js and compiled by webpack, problem solved

[CLAassistant]

All committers have signed the CLA.

[cheekyshibe]

Please keep the change in d3b5f1b pydata_sphinx_theme/docs-navbar.html (line 46)

      <ul class="navbar-nav">
        <li class="version_switcher nav-item dropdown">
          {%- include "version-switcher.html" %}
        </li>
      </ul>

[cheekyshibe]

If version.json can not be found. It would be better to hide the version switcher component.