Degrade gracefully in the absence of the jQuery waypoints plug-in

When the plug-in is not available, we just revert to our old,
eager-loading ways.

Related issue:
  https://github.com/causes/replacejs/issues/2
  ("Move lazy-load functionality into replacejs repo")

Change-Id: I2675f1f5866d2cadf0fdb55e24e91aa5ed7ead66
Reviewed-on: https://gerrit.causes.com/25811
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>
Remove ajaxReplace() from API

Now all the callers of ajaxReplace() are internal to replace.js itself
(there are two call-sites in causes.git that will need to be updated
before this commit is pulled in), which means we can reduce the API
surface area by making ajaxReplace() an internal-only function.

Related issue:
  https://github.com/causes/replacejs/issues/2
  ("Move lazy-load functionality into replacejs repo")

Change-Id: Iab276c00d3524a06546e066ddd88fa4a4441c2c1
Reviewed-on: https://gerrit.causes.com/25812
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>