add feature which allow related links to be added

Implemented a feature which allows related links to be attached when editing a task or adding comments under the task. In the back end, added two functions called upload_links and delete_links which insert and delete the links into mysql database table flyspray_links. In the front end, modified the templates details.edit.tpl, details.view.tpl, editcomment.tpl, details.tabs.comment.tpl, added the lines to display the interface when users are editing or viewing the links. Also added two javascript functions addLinkField and removeLinkField in functions.js so that users can dynamically add and remove links before the results are saved. Finally, two new templates called common.links.tpl and common.editlinks.tpl are created in order to display the links either in the view or edit mode. Other modifications are made to adjust to the changes made above.