#137 Feature/template utilities

[richaber]

Per #137

Start breaking some of the template responsibilities out into a Utilities class.
This is prep work for comparing customized template versions against versions that ship with the plugin.
This will be used for admin notices that warn users if their customized template versions are out of date.

• Add Algolia_Template_Utils class
• Rename TEMPLATES_DIR to PLUGIN_TEMPLATES_DIR
• Require the Algolia_Template_Utils file
• Deprecate Algolia_Template_Loader::locate_template and return Algolia_Template_Utils::locate_template
• Return Algolia_Template_Utils::locate_template in Algolia_Template_Loader::load_autocomplete_template
• Return Algolia_Template_Utils::locate_template in Algolia_Template_Loader::load_instantsearch_template
• Clarify template locations in Algolia_Template_Loader::template_loader docblock
• Deprecate algolia_templates_path filter
• Deprecate Algolia_Plugin::get_templates_path method
• Algolia_Template_Loader::load_autocomplete_template should require not return

[asharirfan]

LGTM 👍