Add translation keys for sign-in page (#1551)

* Begin Work

* WIP

* WIP

* WIP

* Use error notification keys

* Finish & fix error

* Fix lint error

* Normalize message IDs (#9)

It makes sense to compose message IDs in order:
- Place (page, sub page / "modal")
- Thing
- (optionally) Relation to the thing

For example, a label for a password field would be:
- auth.sign-in (on sign-in subpage of auth)
- password (password field)
- label (is a label for the field)

Another example - button to sign in:
- auth.sign-in
- action (this is an action to do something)
- sign-in (action to sign in)

This helps keep the IDs closer to the actual structure of the page,
oftentimes smaller in the code, and easier to understand by translators.

---------

Co-authored-by: Sasha Sorokin <10401817+brawaru@users.noreply.github.com>
Co-authored-by: Prospector <6166773+Prospector@users.noreply.github.com>

locales/en-US/index.json

@@ -1,4 +1,34 @@
 {
+  "auth.sign-in.2fa.description": {
+    "message": "Please enter a two-factor code to proceed."
+  },
+  "auth.sign-in.2fa.label": {
+    "message": "Enter two-factor code"
+  },
+  "auth.sign-in.2fa.placeholder": {
+    "message": "Enter code..."
+  },
+  "auth.sign-in.action.sign-in": {
+    "message": "Sign in"
+  },
+  "auth.sign-in.additional-options": {
+    "message": "<forgot-password-link>Forgot password?</forgot-password-link> • <create-account-link>Create an account</create-account-link>"
+  },
+  "auth.sign-in.email-username.label": {
+    "message": "Email or username"
+  },
+  "auth.sign-in.password.label": {
+    "message": "Password"
+  },
+  "auth.sign-in.sign-in-with": {
+    "message": "Sign in with"
+  },
+  "auth.sign-in.title": {
+    "message": "Sign In"
+  },
+  "auth.sign-in.use-password": {
+    "message": "Or use a password"
+  },
   "auth.sign-up.action.create-account": {
     "message": "Create account"
   },