Prevent collision naming with a gitea-vendor directory

[camlafit]

Hello

To note a discussion with @sapk discord, could be interesting to use a root directory as gitea-vendor/ or web-vendor/ or any other path to store all statics resources.

Could be solve with a different way #20

Actual solution has two troubles :

• we must manage an hardcoded reserved keyword (an some name could be licit account/organization)
• An 500 error is raised without any information when we try to rename an organization with a keyword in this list.

[wxiaoguang]

/assets is reserved and it's for static resources.