[feature] Nicer login.html page, with cute styling + link to source code (#19)

Fixes #9

- Cute styling, combination of Mastodon and GTS
- Short description of the project
- Error and status messages (temporarily) appear in disabled button with correct ARIA attributes
- Sufficient contrast (WCAG AAA)

Let me know if using `login.scss` both as an index file and for adding custom styling is okay. I figured this might be preferred over creating an extra folder and file.

Reviewed-on: https://codeberg.org/superseriousbusiness/masto-fe-standalone/pulls/19
Co-authored-by: vyxen <vyxen@tutamail.com>
Co-committed-by: vyxen <vyxen@tutamail.com>

app/javascript/flavours/glitch/theme.yml

@@ -16,6 +16,9 @@ pack:
       - flavours/glitch/async/getting_started
       - flavours/glitch/async/home_timeline
       - flavours/glitch/async/notifications
+  login:
+    filename: packs/login.js
+    stylesheet: true
   mailer:
   modal:
   public: packs/public.jsx