Allow components to specify javascript or style sheets

Currently, if a component (custom field etc) wants to include javascript or css, it has to manually include it in the page. This increases the page load time.

Another problem is that if javascript is needed on a few pages, there is a tendency to just include it at the top of all pages. (eg the javascript for the wysiwyg editor). Including all the javascript on the first page has the effect of making JIRA appear slow.

Components should be able to specify that they need javascript / css, and it be rendered at the top of the page.

One way in which this could be implemented is using content tags inside the html, and then in the decorators, pulling out the content tags, and specifying the javascript that is needed. That way if more than one content tag was specified, then only one file would be served. This is a clean separation of concerns, and also reduces the amount of bandwidth that is needed.