Update to JHipster 7.8.1

[mshima]

• Reimplement generators from the ground.
• Keep jhipster dependencies at 3.9.1 (latest jhipster 6).
  Migrate everything at once would be too challenging.
• Use the jhipster domains (entities), and dto support.
  Should make maintenance easier.
• Use jhipster liquibase files
• Customize client frameworks files instead of using our own templates for every change.
  Most of it, some pages still needs lots of changes, so keep the custom templates.
• Adapt api breaking changes: update (put with id), not flatten dto.
• Adapt templates.
• Fix some bugs like save current user info and exception handling.
• Fine tune apis behavior to match jhipster.(response status, accepted content type)
• Not used templates are kept in templates_back folder for reference. Can be removed later.

[atomfrede]

Thats awesome @mshima. I will try to test it during the weekend, but as all tests pass I tend to merge it and continue the work to update micronaut etc.

[atomfrede]

Going to merge it as it seems good to me, reviewing every one of the 188 files won't work. Thank you very much @mshima, please don't forget to claim the bounty