Website download/verify page update for 0.7.0-M2

Rewrites the website download page, and moves the verify page from the
guide to the website.
Use Brooklyn version from metadata if appropriate

Responding to review comments. Prefer using {{
site.data.brooklyn.version }} instead of 0.7.0-M2-incubating in most
places - exceptions are where we are keeping metadata for specific
versions e.g. in the hash download links
Bump BROOKLYN_VERSION in metadata plugin
Further website updates for new version

Corrects the server install guide and quickstart guide links for the M2
release
Merge and close #420