[frontend] Fix #923 and markdown css (#925)

* fix url in CONTRIBUTING.md

* prevent img from overflow and allow to center the img

* fix #923 and guide browser to select entire username

web/source/css/profile.css

@@ -126,6 +126,7 @@ main {
 			word-break: break-all;
 			text-overflow: ellipsis;
 			overflow: hidden;
+			user-select: all;
 		}
 	}
 

web/source/css/status.css

@@ -178,6 +178,14 @@ main {
 					-webkit-overflow-scrolling: touch;
 				}
 			}
+
+			img {
+				max-width: 100%;
+				margin: 5px auto;
+			}
+			img[alt~="!center"] {
+				display: block;
+			}
 		}
 
 		.emoji {