Add back underlines to links in posts (#1386)

As mentioned [in Hometown's
README](https://github.com/hometown-fork/hometown#better-accessibility-defaults),
links in posts are meant to be underlined by default for better
accessibility. However, this default was overridden in one of the
stylesheets so [every Hometown instance I know
of](https://github.com/hometown-fork/hometown/wiki/Hometown-servers)
(with the exception of weirder.earth) does not have any underlined links
in posts as of right now. This pull request restores those underlines.

Before (feed view):
<img width="579" height="183" alt="The post link has no underline."
src="https://github.com/user-attachments/assets/7dc3d5bb-dca4-4dcd-9148-98c343a68316"
/>

After (feed view):
<img width="579" height="183" alt="The post link has an underline now."
src="https://github.com/user-attachments/assets/5b892a89-f252-4ccb-925a-76d410fa426e"
/>

Before (expanded view):
<img width="582" height="223" alt="The post link has no underline."
src="https://github.com/user-attachments/assets/8e41e0e7-7fca-43f5-a5d5-bf0147d3a9cd"
/>

After (expanded view):
<img width="582" height="223" alt="The post link has an underline now."
src="https://github.com/user-attachments/assets/5c209ec4-690a-4e74-a90e-dfc4eca44867"
/>

app/javascript/styles/mastodon/components.scss

@@ -1188,7 +1188,7 @@
 
   a {
     color: $secondary-text-color;
-    text-decoration: none;
+    text-decoration: underline;
     unicode-bidi: isolate;
 
     &:hover {