alert: Make selectors less overly specific and clean up js

Follows-up 383a818.

* There is no need for the additional element ("a") or
  descendant ("#pt-mytalk") selector.
  It isn't overrriding anything, only hardcodes details that make
  it harder to maintain or extend this stylesheet. For example,
  there is a gadget that makes  user messages green instead of
  orange, it now was required to hardcode the "#pt-mytalk a" part
  of the selector eventhough those are subject to change.
  Separation of concerns.

* Cache/reuse the jQuery object instead of executing the same
  query to the document, again. It also avoids a potential bug
  where the second query matches different elements (e.g. after
  appending alertMessage, there could potentially be an additional
  nested anchor link; there isn't now though, as the message is
  plain text).

* Add comment about weird echoNewMsgAlertDisplayed variable.

Change-Id: I682182fe15a868969f25fa5bfe2412e2a6f3dddf
Merge "alert: Make selectors less overly specific and clean up js"