[#6393] Load macros from external packages

* Moved download_button macro out of allura
* Fixed/cleaned up tests that rely on pylons globals

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
[#6393] Cleanup

* Removed download button test
* Changed show_download_button macro default to False
* Added docs for extending Allura with macros

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
[#6393] Remove download button from default wiki text

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>