Change the moment we check for activate/deactivate features

[Rahmon]

Description of the Change

This PR changes the moment we check for activate/deactivate features updating the hook from init to admin_init on the class Features.

Alternate Designs

Benefits

Possible Drawbacks

The check for activate/deactivate features will not run on WP-CLI commands.

Verification Process

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Closes: #2362

Changelog Entry

[Rahmon]

@felipeelia I changed a little bit the initial approach in this PR. Moving the functions handle_feature_activation and setup_features from the hook init to admin_init break some tests because inside those functions other functions are added to the hook init.

So, instead of moving to admin_init, I'm checking if is_admin() where we change the statuses of the features.

[felipeelia]

This is actually a comment we should keep, shouldn't we @Rahmon ?

[Rahmon]

Yes. I added back 😬

[felipeelia]

As is, this seems to be breaking WP-CLI index. These are the steps to reproduce the problem I'm seeing @Rahmon:

1. Create a multisite, enable EP for the entire network and WooCommerce just on the main site
2. Run wp elasticpress index --setup --network-wide --yes
3. Visit /?s=woo-hoodie&post_type=product in the main site and see 3 products (found by SKU)
4. Visit /wp-admin/plugins.php?plugin_status=all&paged=1&s in the second site (the one without WC)
5. Run wp elasticpress index --setup --network-wide --yes again
6. Visit /?s=woo-hoodie&post_type=product in the main site and see no product found

It seems #4 removes the WC feature (what would be expected, as WC is not available on that site), and then on #5 (as the WC feature is inactive) SKUs are not indexed.

[Rahmon]

I've updated to check for a WP-CLI context