Rename plugin to WordPress Learn App

[akirk]

Summary

• rename the plugin bootstrap to learn-app.php and update plugin/readme metadata
• move course plan storage into CoursePlans and Learn WordPress API access into LearnApi
• update app routes and blueprint install metadata to use /learn-app/ and the learn-app repository

Testing

• php -l learn-app.php
• php -l src/App.php
• php -l src/CoursePlans.php
• php -l src/LearnApi.php
• php -l templates/index.php
• php -r 'json_decode(file_get_contents("blueprint.json"), true); if (JSON_ERROR_NONE !== json_last_error()) { fwrite(STDERR, json_last_error_msg() . PHP_EOL); exit(1); } echo "blueprint.json valid JSON\n";'