chore(tagcloud): move site tagcloud functionality to the tagcloud plugin

fixes #7140
chore(tagcloud): moved tagcloud theme sandbox output to tagcloud plugin
chore(tagcloud): moved tagcloud related views to tagcloud plugin
chore(tagcloud): moved tagcloud related language strings to plugin
chore(tagcloud): moved tagcloud related css to tagcloud plugin
Merge pull request #7510 from jdalsem/move-tagcloud-out-of-core

Move tagcloud out of core