Internationalization and accessibility sanity

* Moving to standardize on using <?php and ?> of codeblocks
* Centralized all app strings in press-this.php::WpPressThis::i18n()
method to only have to maintain app strings in one single place
* Moved the latter to be based on keys we can keep when strings change
* removed unnecessary press_this_ prefix from some methods and moved
some in more logical locations
* moved to using a[title] instead of a[aria-label] as it’s just as good
for accessibility since used for a tags, but also gives all users
tooltips.