Standardize WPJM function and hook names

The prefixes used and internal naming of things is inconsistent or nonexistent. In a major release, we should come up with a standard naming scheme, deprecate usage of the old functions/hooks, and phase in the new standard.

Seems like `wpjm_` is the winning prefix for functions/hooks. `WP_Job_Manager_` for class names, as it seems to be the most used prefix already.

I'd like to start up a CONTRIBUTING file to document some of this (#955).

Start by reviewing the existing function, hook, and class naming patterns across the plugin, then read issue #955 for the proposed CONTRIBUTING documentation. Done means agreeing on a naming standard, identifying old APIs for deprecation and phased migration, and documenting the conventions; the issue does not name specific files or tests.

Written by the indexing model from the issue text.