Add LibreJS support

[strk]

This was added in Gogs on February 8 2017:
gogs/gogs#4092

Would be good to have that PR ported to Gitea too.

[tboerger]

I still got the opinion that this is a pretty useless feature.

[lunny]

Why we need add this lib?

[strk]

@tboerger it is as useful as having a LICENSE file in the Gitea source code, and all those copyright info.

[strk]

NOTE: there's no lib being added, just information about the license covering the Javascript code distributed by the service