cylc-7 and cylc-8 download buttons

[hjoliver]

Close #34

• two download buttons that clearly indicate "production" or "preview" use
• (removes the automatically updating list of multiple latest releases ... it was kind of cool, but used up screen real estate and no one really needs it).
• (also gratitiously tidied some front page text a bit)

Thought I'd better get this done before going home, in the interests of minimizing user confusion, since @sadielbartholomew raised #30 .

@kinow - my JS may be a bit ropey, but it seems to work https://hjoliver.github.io/cylc.github.io/

One review should do.

[kinow]

Looks good!

Nit-picking on the tabs vs. spaces (again hehe), but pre-approved, as this is not a blocker IMO.

[matthewrmshin]

Some suggestion. Feel free to ignore or postpone. Does what it says otherwise.

[sadielbartholomew]

This (builds &) looks great & as described, plus it's a nice touch that the user sites logo set links through to the users page. Having:

two download buttons that clearly indicate "production" or "preview" use

seems like a very good solution to the current scenario regarding expected/future releases.

One review should do.

.. but have three 😄

(I'm not going to merge since Matt has raised a very good suggestion, so I'll approve & leave you to decide whether you would like to address that & if so, in this PR. Please merge when you see fit.)