Make subscriptions handles reactively stop.

Although `handle.ready()` became false when the subscription was stopped, it didn't do so reactively, 
which led to some difficult to understand issues.
Added test case for reactive handle argument changing.
Explicitly test for sub arguments changing before ready.
Ensure the sub reactively goes unready when removed.

Subscriptions are removed from 3 points - handle.stop, nosub message, and _unsubscribeAll. We need to handle all three cases similarly.
Removed redundant checks for Deps' existence.
Subs’ readiness should not invalidate if they stop when they aren’t ready.
{{> UI.dynamic }} for easier dynamic template picking.

See #2123.
Drop collections after test.

Only on the consistent id test, but that one is run repeatedly.

Written using cleanup functions; we might want to make this part
of tinytest in future.
Switch the 'unreachable' url from http://asdf.asdf to http://0.0.0.0

Some ISPs / proxies serve a valid HTTP response on any DNS miss, but
we are trying to test an unreachable server.
Expose underscored animation hooks and add a test
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
Update an accounts-ui-unstyled comment
Show name of logged-in user after email verification/password reset
Merge branch 'login-display-username' into devel
Rename `closePopup` to `OAuth._endOfLoginResponse`.