MBO Refactor alerting code

[adamfowleruk]

files lib-search-subscribe and lib-alerts

• remove eval's, replace with invoke function
• remove unnecessary code (was copied from another project originally)
• reduce down to one utility file (lib-alerting.xqy?)
• Keep final file generic (not limited to just MarkLogic Workflow uses, much like lib-stack.xqy)

Note: Files beginning with lib- are generic, ones starting with workflow- are specific to MarkLogic Workflow.