Fixed a JavaScript syntax error.

Fixed magnet tab opening logic

Fixes provided by user:
* Prevent redefinition error and being able to click more than once an reprocess each tab
* Support Waterfox Classic
* Modification to loop over all tabs instead of processing current tab
* Cleanup the buffer to not have the link created multiple times

Fix bug causing previously opened links to always stay in memory. Thanks to Alexandre TREME for reporting the issue.

* Upgrade to WebExtension. A lot of bells and whistles were lost, working on it!