Revert "fix weirdness in RawHtml mention regex"

This reverts commit f4710dc7f8.

> This was intended to prevent backtracking.
{2,30}+ is posessive, i.e., it will not give back any characters it has consumed.

modules/common/src/main/base/RawHtml.scala

@@ -42,7 +42,7 @@ final object RawHtml {
   // word (i.e. preceded and followed by space or appropriate punctuation):
   // Yes: everyone says @ornicar is a pretty cool guy
   // No: contact@lichess.org, @1, http://example.com/@happy0, @lichess.org
-  val atUsernameRegex = """@(?<![\w@#/]@)([\w-]{2,30})(?![@\w-]|\.\w)""".r
+  val atUsernameRegex = """@(?<![\w@#/]@)([\w-]{2,30}+)(?![@\w-]|\.\w)""".r
 
   private[this] val atUsernamePat = atUsernameRegex.pattern