(core) User language switcher

Summary:
New language selector on the Account page for logged-in users.
New icon for switching language for an anonymous user.

For anonymous users, language is stored in a cookie grist_user_locale.
Language is stored in user settings for authenticated users and takes
precedence over what is stored in the cookie.

Test Plan: New tests

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3766

static/icons/locales/DE.svg

@@ -0,0 +1,10 @@
+<svg width="16" height="12" fill="none" xmlns="http://www.w3.org/2000/svg">
+  <mask id="a" maskUnits="userSpaceOnUse" x="0" y="0" width="16" height="12">
+    <rect width="16" height="12" rx="-1" fill="#fff"/>
+  </mask>
+  <g mask="url(#a)" fill-rule="evenodd" clip-rule="evenodd">
+    <path d="M0 8h16v4H0V8z" fill="#FFD018"/>
+    <path d="M0 4h16v4H0V4z" fill="#E31D1C"/>
+    <path d="M0 0h16v4H0V0z" fill="#272727"/>
+  </g>
+</svg>