users: refactor password reset flow.

The old password reset flow:

* User requests a password reset.
* Danbooru generates a password reset nonce.
* Danbooru emails user a password reset confirmation link.
* User follows link to password reset confirmation page.
* The link contains a nonce authenticating the user.
* User confirms password reset.
* Danbooru resets user's password to a random string.
* Danbooru emails user their new password in plaintext.

The new password reset flow:

* User requests a password reset.
* Danbooru emails user a password reset link.
* User follows link to password edit page.
* The link contains a signed_user_id param authenticating the user.
* User changes their own password.

app/controllers/password_resets_controller.rb

@@ -0,0 +1,14 @@
+class PasswordResetsController < ApplicationController
+  respond_to :html, :xml, :json
+
+  def create
+    @user = User.find_by_name(params.dig(:user, :name))
+    UserMailer.password_reset(@user).deliver_later
+
+    flash[:notice] = "Password reset email sent. Check your email"
+    respond_with(@user, location: new_session_path)
+  end
+
+  def show
+  end
+end