Fix parsing encoded matrix.to URL (#660)

From https://spec.matrix.org/v1.3/appendices/#matrixto-navigation:

	The components of the matrix.to URI (<identifier> and <extra parameter>) are to be percent-encoded as per RFC 3986.

	Historically, clients have not produced URIs which are fully encoded. Clients should try to interpret these cases to the best of their ability. For example, an unencoded room alias should still work within the client if possible

src/util/sanitize.js

@@ -44,7 +44,7 @@ function transformSpanTag(tagName, attribs) {
 }
 
 function transformATag(tagName, attribs) {
-  const userLink = attribs.href.match(/^https?:\/\/matrix.to\/#\/(@.+:.+)/);
+  const userLink = decodeURIComponent(attribs.href).match(/^https?:\/\/matrix.to\/#\/(@.+:.+)/);
   if (userLink !== null) {
     // convert user link to pill
     const userId = userLink[1];