add froala

[3xxx]

it can copy word images and text into mindoc, images can autosave to mindoc server.
it can copy web images and text into mindoc, images can autosave to mindoc server.
before use froala,you shoud copy froala css and js files in mindoc static/
then in the setting,change to froala editor than save.

[gsw945]

先 merge 了
模板中 静态资源 url 建议写成形如 {{ cdncss "/static/xxx.js" }} 的方式，防止将 mindoc 反向代理到二级目录后，资源加载异常

[3xxx]

是的。下次改过来。