Remove long-deprecated instructions about promethesus console; also fix for modern config file format

Not logical to use svn checkout against a github repo, so changed to git clone. 

Signed-off-by: Rick Cogley <rick.cogley@esolia.co.jp>

svn checkout is not logical for a checkout from github, so changed the checkout to "git clone". 
thanks @dbkr

Signed-off-by: Rick Cogley <rick.cogley@esolia.co.jp>

Reading the RST spec, I was trying to get breaks to appear by entering the double spaces after the lines in the code blocks. It does not work anyway, and, as pointed out, I've removed.

Additionally it does not appear there is turnserver.conf.default, but rather, just /etc/turnserver.conf.

initial WIP of a tentative preview_url endpoint - incomplete, untested, experimental, etc. just putting it here for safekeeping for now