SCSS: merge %links-text and %lists-text

These were always being used together, so there's no real need to have
different placeholders.

tildes/scss/_base.scss

@@ -43,8 +43,7 @@ code {
 }
 
 dl dd {
-  @extend %links-text;
-  @extend %lists-text;
+  @extend %text-container;
 
   margin-left: 0.4rem;
   font-size: 0.7rem;

tildes/scss/_placeholders.scss

@@ -1,8 +1,8 @@
 // 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 {
+// styles specifically for elements containing text content (often from markdown)
+%text-container {
   a {
     text-decoration: underline;
 
@@ -10,10 +10,7 @@
       text-decoration: none;
     }
   }
-}
 
-// sets up (nested) lists specifically for text content
-%lists-text {
   ol, ul {
     margin: 0 0 0.4rem 1rem;
 

tildes/scss/modules/_comment.scss

@@ -135,8 +135,7 @@
 }
 
 .comment-text {
-  @extend %links-text;
-  @extend %lists-text;
+  @extend %text-container;
 
   padding: 0.2rem;
   padding-left: 0.4rem;

tildes/scss/modules/_form.scss

@@ -45,8 +45,7 @@ select.form-select:not([multiple]) {
 }
 
 .form-markdown-preview {
-  @extend %links-text;
-  @extend %lists-text;
+  @extend %text-container;
 
   min-height: 8rem;
   padding: 0.4rem;

tildes/scss/modules/_message.scss

@@ -24,8 +24,7 @@
 }
 
 .message-text {
-  @extend %links-text;
-  @extend %lists-text;
+  @extend %text-container;
 
   margin-left: 0.2rem;
   overflow: auto;

tildes/scss/modules/_static-site.scss

@@ -8,7 +8,6 @@
 
 body.static-site {
   main {
-    @extend %links-text;
-    @extend %lists-text;
+    @extend %text-container;
   }
 }

tildes/scss/modules/_topic.scss

@@ -156,8 +156,7 @@
 }
 
 .topic-text-excerpt {
-  @extend %links-text;
-  @extend %lists-text;
+  @extend %text-container;
 
   display: none;
   @media (min-width: $size-md) {
@@ -248,8 +247,7 @@
 }
 
 .topic-full-text {
-  @extend %links-text;
-  @extend %lists-text;
+  @extend %text-container;
 
   overflow: auto;
 }