[BUGS-5161] Update deprecated filter call

This PR resolves the issue reported in BUGS-5161 as well as making a number of other minor updates that improve the overall quality of the Solr Power plugin.

The commit that resolves the actual bug is 9e259f8
- The whitelist_options filter name was deprecated in WP 5.5 in favor of the more inclusive allowed_options. The resolution is simply using the new filter name.
Unrelatedly, my first quality-of-life improvement was to replace sanity_check with environment_check for similar reasons e296885
When using the plugin locally, if SOLR_PATH is not defined, on activation, the plugin assumes you want to upload a schema. This is likely not the case, but the error message does not tell you what's wrong. This PR updates the error message to indicate that SOLR_PATH is not defined, so if that's the problem, you might know how to fix it. 8a5cbc9
If the plugin is installed on a WordPress multisite, the docs indicate that you should Network Activate the plugin, but that is not enforced. 3f53c37 changes this to wp_die if you activate on a single site of a multisite and links to the Network Admin Plugins page to network activate there.
Grunt actions were not running because of changes to grunt-sass. This PR updates the package.json to require node-sass and implements this in the Gruntfile.
Changes were necessary in the Gruntfile which was calling a parameter that was not (or no longer) supported, resulting in errors when Grunt was run.
Some changes were made to the PHPCS rules to not flag short array syntax ([]) or warn about not aligning equals signs.
- Short array syntax was added in PHP 5.4, so there's no reason to require arrays to be written in long form.
- Aligning equals signs for variables require additional time (that could be better used elsewhere on more important tasks) to align and re-align variable assignments when new variables are added or renamed.

fixes #541 related #538

pantheon-systems / solr-power

[BUGS-5161] Update deprecated filter call #544