Remove UI.destroy

Don’t fix the docs yet; will do a pass that includes removing UI.insert
All-but-remove the `ui` package

Also improve info strings on blaze and ui

Not in this commit:
* Change all uses of `ui` package to use `blaze`
* Change all references to UI symbol to Blaze
* Change docs accordingly
Fix infinite reload loop running tests

(just a guess at the fix; arbesfeld to fix the fix if necessary)
Merge branch 'refs/heads/devel' into blaze-templates
Rename references to ‘ui’ package to ‘blaze’