New and complete Spanish localization files

This is a complete and new Spanish localization – 100% of 129 lines translated — replacing previous incomplete files which had only 29 lines in it. New one is based on the provided .pot template file and includes translation for plugin elements, and Wordpress Admin UI elements.

Derived from reference and discussion #175. What appeared as multiple changes in that [diff](https://github.com/Automattic/Co-Authors-Plus/commit/84ac540dc4d0f173c51799c0b5c8a6b3b6ac93d6) was actually just line breaks.