FOSSBilling v0.8 compatibility and improvements

[yagiz-dev]

WIP

[getpinga]

Thank you for the contribution and for taking the time to prepare this PR.

Unfortunately, we cannot accept the PR in its current form. The changes in the html folder are valuable and appear to fix compatibility issues with FOSSBilling 0.8.2. However, the removal of the assets would break several workflows that are currently used by other components of the Namingo project.

Would it be possible to split this into two separate PRs? One containing only the changes to the html folder for the FOSSBilling 0.8.2 compatibility fixes, and another containing the remaining changes. This would allow us to review and potentially merge the compatibility fixes independently and consider the other changes for future versions.

Thank you again for your contribution and understanding.