Use Browser or OS light/dark theme preference

Change i18n from logical keys to "textual" ones, add app localization, add PT/ES localizations

These changes allow the user to have localization in the desktop
application (menu) and to change it from the Headlamp's frontend.

Also changes the namespace separator key from ":" to "|" to avoid
parsing issues from i18next.

If we are not using the locales (because they're incomplete), let's not
ship them at all.

This commit adds i18next-parser and a few convenience commands in order
to generate/create the translation files for the existing locales.

For creating a new locale, create the locale directory under
frontend/src/i18n/locales and run `make i18n`.

Change cluster modal to focus first cluster button

If the user makes a preference by selecting the theme from the UI,
then it uses that preference instead. Once the user preference is
selected it always uses that preference from then on.

The localStorage key was prefixed with headlamp to try and avoid
nameclashes. The older key was ignored, since it was set without
a user selecting a preference. This way the browser/OS preference
is used until they change it manually.

Also make the theme preference change reactive. So it changes
when the os/browser preference changes.

Fix editor, which was breaking after typing

Humble a11y fixes

By request: https://github.com/kinvolk/headlamp/pull/244#issuecomment-852075724

docs: Add EKS to the list of tested platforms that work

As reported in https://github.com/kinvolk/headlamp/issues/266 , let's add EKS as "works".

The Editor before could trap focus, meaning tab would not escape
the editor. So we give the option to use a simple textarea
based editor.