Downloads page rework

[hinerm]

Proposed changes for http://imagej.net/Downloads

• [ ] Create "Distributions of ImageJ" page (template?)
• [x] Move Fiji downloads to fiji.sc
• [x] Add links to distribution pages below primary ImageJ download
• [ ] Add JRE bundled downloads

[ctrueden]

Progress on this:

• http://imagej.net/Downloads = vanilla ImageJ2 downloads page
• http://imagej.net/Fiji/Downloads = Fiji downloads page
• http://fiji/sc/#download = Fiji splash page, Download section
• http://fiji.sc/<Foo> = blanket redirect to http://imagej.net/<Foo>

And the main/vanilla Downloads page has a link to the Fiji downloads page underneath the main ImageJ2 download.

Still to do:

• Make Jenkins job that builds latest vanilla ImageJ2 for each platform
• Serve those from the main Downloads page, analogous to how Fiji downloads are structured
• Probably flesh out and generalize the "Distributions of ImageJ" to encompass other distributions besides Fiji such as BoneJ