Avoid exceptions in groovy views; check for existence of build workspace

- On the integration status page, an exception is generated if there is no currently-integrated version (moved the line that prints the current version inside the block executed only if ia != null).
- On the integration status page, the l.main_panel closure ends too early, causing the integration status text to be displayed incorrectly.
- In the rebase and integrate actions, check to ensure that there is a build with a workspace available to use as a working copy for the merge actions.  Since the workspace is fetched via owner.getSomeBuildWithWorspace().getModuleRoot(), I think these fixes should account correctly for workspaces on nodes other than the master, but I'm not entirely sure.