Add support for SSH-style git repo addresses (#835)

This allows nodes to be pulled from git repositories that:
1. Require authentication
2. Are located on a computer that does not have any special software
   beyond an SSH server to serve up git repos
3. Are hosted on sites that exclusively allow SSH access

I have also cleaned up the JavaScript regex for identifying valid HTTP
addresses. Due to an unescaped '.' and the lack of a count on the first
group, it wasn't doing a whole lot anyway -- just checking that the very
first character wasn't invalid.

js/common.js

@@ -34,8 +34,9 @@ function isValidURL(url) {
 	if(url.includes('&'))
 		return false;
 
-	const pattern = /^(https?|ftp):\/\/[^\s/$.?#].[^\s]*$/;
-	return pattern.test(url);
+	const http_pattern = /^(https?|ftp):\/\/[^\s$?#]+$/;
+	const ssh_pattern = /^(.+@|ssh:\/\/).+:.+$/;
+	return http_pattern.test(url) || ssh_pattern.test(url);
 }
 
 export async function install_pip(packages) {