Move SJCL license file into core plugin

It was added as a separate system tiddler, but that means updating a
lot of filters to add and exclude it as appropriate (as we do with
$:/library/sjcl.js itself)

boot/sjcl-license.txt.meta

@@ -1,2 +0,0 @@
-title: $:/library/sjcl.js/license
-type: text/plain

core/sjcl-license.tid

@@ -1,3 +1,6 @@
+title: $:/library/sjcl.js/license
+type: text/plain
+
 SJCL is open. You can use, modify and redistribute it under a BSD
 license or under the GNU GPL, version 2.0.