Fix flash of unlinked template when changing groups

When an app reload occurs, the 'Clear Search' and
'Clear Selection' buttons would sometimes flash.

This occurred due to an issue where the initial
call to the watcher registered by ng-show to hide
the buttons being invoked asynchronously in some cases.

When the app's route is loaded, the following happens:

 1) viewer.html is compiled
 2) viewer.html is linked
 3) Watchers registered by directives during (1) and (2)
    are run. For the watchers registered by 'ng-show',
    this applies the .nghide CSS class that hides
    elements.

On initial app load, 1-3 all happen synchronously in the
same scope.$digest cycle. However, when logging in with
an incorrect password, 1 & 2 happened in the same cycle
but 3 happened in a separate cycle, with DOM rendering
taking place in a flash between before the directive
was fully ready.

The GitHub issue has more detail and there is some
connection to the 'deepCount' directive but I decided
not to alter that here without sufficient understanding
of the consequences.

This commit fixes the issue by applying the 'nghide'
class to the viewer.html initially and then letting
ng-show _remove it_ when its watcher runs. This fixes
the flash when 1-2 and 3 happen in separate digest cycles
and has no effect if they run in the same cycle.

Fixes #2642

h/templates/client/viewer.html

-<!-- Thread view -->
-<ul class="stream-list"
+<!-- Annotation thread view
+
+(See gh2642 for rationale for 'ng-show="true"')
+ -->
+<ul class="stream-list ng-hide"
+    ng-show="true"
     deep-count="count"
     thread-filter="search.query"
     window-scroll="loadMore(20)">