Implementing a project admin UI in a plugin is broken

tl;dr: if a page with an admin decorator is hooked to a project admin plugin point (either a tab, or in the Actions menu), it tries to render a project admin specific header

I recently tried to implement a project admin UI that fits in well with the recent Ignite UI changes in my issue collector plugin. This is a lot harder than one might think!

The bug in particular I'm raising here is that the header web-panel for the project admin decorator expectes the project to be set in ProjectConfigRequestCache. This however only gets set if you go through the project admin PanelServlet. If I'm merely using the project admin decorator by including the correct meta tags in my page then the project doesn't get set in the ProjectConfigRequestCache and you get an IllegalStateException and massive stack trace in the logs any time your project admin page is rendered. We should fix this so that the project gets picked up via meta tags and doesn't have to be set by the PanelServlet!