feat(editor): add external link warning page

A malicious user could modify a valid looking link (for example of the editor UI)
to point to a malicous site, by using overlays and CSS. Since CSS should stay
enabled, we need to make the user aware of possible risks when leaving the
HedgeDoc instance, in order to protect them from credential-theft.

This commit adds a new interstitial page for external links, that shows the
target URL and asks the user, whether they really want to continue.

Signed-off-by: Erik Michelson <github@erik.michelson.eu>

locales/de.json

@@ -126,5 +126,13 @@
     "Protected - Only owner can edit (forbid guests)": "Geschützt - Nur Besitzer kann bearbeiten (Gäste verboten)",
     "Private - Only owner can view & edit": "Privat - Nur Besitzer kann anschauen & bearbeiten",
     "changed": "verändert",
-    "created": "erstellt"
+    "created": "erstellt",
+    "External link": "Externer Link",
+    "You've clicked a link that points to an external page.": "Es wurde ein Link zu einer externen Seite angeklickt.",
+    "The content of that page is unrelated to HedgeDoc. Do not enter your HedgeDoc credentials there.": "Der Inhalt dieser Seite ist unabhängig von HedgeDoc. Dort keine HedgeDoc-Zugangsdaten eingeben.",
+    "Do you want to continue?": "Fortfahren?",
+    "Continue to external page": "Weiter zur externen Seite",
+    "Back to your note": "Zurück zur Notiz",
+    "Invalid link": "Ungültiger Link",
+    "The provided link is not a valid URL.": "Die angegebene URL ist nicht gültig."
 }