{{> UI.dynamic }} for easier dynamic template picking.

See #2123.
Avi/David comments
Reshuffle packages to allow ui to depend on ui-dynamic-template.

Remove the spacebars-compiler -> spacebars and spacebars-compiler -> ui
dependencies. With those dependencies, ui cannot depend on
ui-dynamic-template, because ui-dynamic-template depends on templating,
whose build plugin depends on spacebars-compiler, who depends on
spacebars who depends on ui.

spacebars-compiler doesn't actually need to depend on ui or spacebars;
it only needs to depend on spacebars to add things to the `Spacebars`
namespace, which this commit does by factoring out the Spacebars export
into a separate `spacebars-common` package.
Change dynamic-tests.* file names
Add helper to check arguments to {{> UI.dynamic}}
Add some comments about UI.dynamic templates
Fix bug where data context was being set from falsey to template
Move ui-dynamic-template into spacebars package
Merge branch 'ui-dynamic-template' into devel