Add latex support

Adds latex highlighting support via https://www.npmjs.com/package/katex
as requested in
https://mattermost.uservoice.com/forums/306457-general/suggestions/10160757-add-latex-support

web/sass-files/sass/partials/_post.scss

@@ -496,6 +496,9 @@ body.ios {
 				border-bottom-left-radius: 4px;
 				@include opacity(.3);
 			}
+            &.tex .katex-display {
+                text-align: left;
+            }
 			code {
 				white-space: pre;
 			}