[FEATURE] Add experience level field

Classes/Controller/JoblistController.php

Classes/Domain/Model/ExperienceLevel.php

Classes/Domain/Model/Job.php

Classes/Domain/Repository/ExperienceLevelRepository.php

Classes/Domain/Repository/JobRepository.php

...iguration/TCA/tx_sgjobs_domain_model_experience_level.php

Configuration/TCA/tx_sgjobs_domain_model_job.php

Configuration/TCA/tx_sgjobs_domain_model_job_application.php

Configuration/TypoScript/Common/setup.typoscript

Resources/Private/Language/de.locallang.xlf

Resources/Private/Language/de.locallang_db.xlf

Resources/Private/Language/locallang.xlf

Resources/Private/Language/locallang_db.xlf

Resources/Private/Language/zh.locallang.xlf

Resources/Private/Partials/Filter.html

Resources/Private/Templates/Joblist/Index.html

.../Public/Icons/tx_sgjobs_domain_model_experience_level.svg

ext_tables.sql