Add underlines to links in posts

I think this is a good idea for a few reasons, including accessibility
(people that have difficulty distinguishing the link color will still be
able to recognize links).

tildes/scss/_placeholders.scss

@@ -1,6 +1,17 @@
 // Copyright (c) 2018 Tildes contributors <code@tildes.net>
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
+// sets up links specifically for text content
+%links-text {
+  a {
+    text-decoration: underline;
+
+    &:hover {
+      text-decoration: none;
+    }
+  }
+}
+
 // sets up (nested) lists specifically for text content
 %lists-text {
   ol, ul {

tildes/scss/modules/_comment.scss

@@ -126,6 +126,7 @@
 }
 
 .comment-text {
+  @extend %links-text;
   @extend %lists-text;
 
   padding: 0.2rem;

tildes/scss/modules/_message.scss

@@ -24,6 +24,7 @@
 }
 
 .message-text {
+  @extend %links-text;
   @extend %lists-text;
 
   margin-left: 0.2rem;

tildes/scss/modules/_topic.scss

@@ -147,6 +147,7 @@
 }
 
 .topic-text-excerpt {
+  @extend %links-text;
   @extend %lists-text;
 
   display: none;
@@ -232,6 +233,7 @@
 }
 
 .topic-full-text {
+  @extend %links-text;
   @extend %lists-text;
 
   overflow: auto;