Remove hard dependency on `kenui`

[pierre]

Have kenui inject itself into the standalone UI, instead of making it a hard dependency here.

• [x] https://github.com/killbill/killbill-admin-ui/blob/841e6fac9df751fab8ae4543dabb2e2c91e4df77/kaui.gemspec#L36 should be removed
• [x] All Kenui related code should be moved to https://github.com/killbill/killbill-email-notifications-ui (the gem should inject itself into screens, etc.)

[KBbitsP]

Hi @tungleduyxyz I have tested this.

• Was able to install the email notifications plugin fine.
• The email notifications settings show up on the account.
• I am able to click on it and configure the events to trigger emails.