Add function_exists('add_action') check

[salcode]

Add function_exists('add_action') check to initialization file. When
loading CMB2 via Composer (and autoloading init.php), the init.php will
be loaded every time. Usually this is not a problem, however if we are
also using Composer to load a PHP development tool (e.g. PHP Codesniffer
(phpcs)), when the development tool runs it is NOT in the context of
WordPress.

Running CMB2's init.php outside of the context of WordPress
results in a fatal error because the function add_action() does not
exist. By adding this function_exists() check, we can avoid this fatal
error when autoloading init.php outside of the context of WordPress.

See #1270

Description

Check that the add_action() function exists before calling it in init.php.

Motivation and Context

Fixes #1270.

Risk Level

This seems to be a relatively low risk change. Since we're checking if the function add_action() exists directly before calling it, the only behavior change we should get is avoiding throwing this fatal error.

Testing procedure

develop tests are failing

The tests on the develop branch (before adding any work of my own) are currently failing.

FAILURES!
Tests: 210, Assertions: 690, Failures: 6.

Adding my changes to master

I cherrypicked the one commit in this PR onto the master branch and ran phpunit there.

All tests passed when my change was introduced into the master branch.

...............................................................  63 / 210 ( 30%)
............................................................... 126 / 210 ( 60%)
............................................................... 189 / 210 ( 90%)
.....................                                           210 / 210 (100%)

Time: 9.44 seconds, Memory: 44.00MB

OK (210 tests, 690 assertions)

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• My code and pull requests meets the Contributing guidelines.

[jtsternberg]

Seems legit. Thank you @salcode

[johnbillion]

I don't believe this change makes much sense. Using Composer to autoload a WordPress plugin with side effects is incorrect and shouldn't be fixed by coding around the side effects in every plugin.

See my comment on the same issue reported against the action-scheduler for my reasoning: woocommerce/action-scheduler#303 (comment)

[salcode]

While I recognize this solution is not ideal, I think it is the most practical approach. In the Action Scheduler thread I've added my thoughts in more detail:
woocommerce/action-scheduler#303 (comment)