Remove useless newline

This is used so that we can make <main> take all the remaining space.
This is "needed" because some pages (such as /tags/) don't have enough content to fill the whole height, making the footer appear in the middle of the page.

assets/css/screen.css

@@ -48,6 +48,8 @@ html, body {
     margin: 0;
     padding: 0;
     height: 100%;
+    display: flex;
+    flex-flow: column;
 
     font-size: 20px;
     line-height: 1.6;
@@ -134,6 +136,7 @@ Used to define how every page is displayed.
 */
 
 main {
+    flex: 1;
     display: block;
     box-sizing: border-box;
     width: 100%;
@@ -340,7 +343,7 @@ Used to style the index/homepage.
 */
 
 .mind-index-header_main {
-    height: 100%;
+    flex: 0 0 100%;
     width: 100%;
     margin: auto;
     display: flex;
@@ -618,7 +621,7 @@ instead of pages ".mind-page*".
 */
 
 .mind-post-header {
-    height: 400px;
+    flex: 0 0 400px;
     width: 100%;
     display: flex;
     flex-direction: column;