Fix avatar alignment in "my account" and users#show

In commit 35659d441, we started using an <svg> tag instead of an <img>
tag to render avatars. That meant that the `vertical-align: middle` rule
we've got for images was no longer being applied.

So we're adding it back.

The only places where this icon wasn't rendered correctly were "my
account" and the show action in the users controller. The comments were
not affected because we've got a `float: left` rule for the
`comment-avatar svg` selector, which causes the browser to ignore the
value of the `vertical-align` propertly, and the avatars showing author
information were not affected because the `.author-photo` selector
already had a `vertical-align` rule.

app/assets/stylesheets/shared/avatar.scss

@@ -1,4 +1,5 @@
 .initialjs-avatar {
   font-family: HelveticaNeue-Light, "Helvetica Neue Light", "Helvetica Neue",
                Helvetica, Arial, "Lucida Grande", sans-serif;
+  vertical-align: middle;
 }