Before: the concept "Content string" is used everywhere. It has some
problems:
1. Sometimes it means "base64 encoded content", sometimes it means "raw
binary content"
2. It doesn't work with large files, eg: uploading a 1G LFS file would
make Gitea process OOM
This PR does the refactoring: use "ContentReader" / "ContentBase64"
instead of "Content"
This PR is not breaking because the key in API JSON is still "content":
`` ContentBase64 string `json:"content"` ``
236c645bf1