Reduce JWT token size

[ishank011]

Based on the problems described in #1935 and the proxy workaround proving to be extremely non-performant, we'll look into reducing the size of the constructed tokens.

The size inflation is because of two objects:

• The user groups: We'll remove the groups from the user object encoded in the token and the services requiring these will make the GRPC call to the userprovider service and maintain a local cache.
• Scopes: We'll try to use some abbreviations for the scopes. Also, to improve performance, I added some code to prematurely expand these for public shares and lightweight accounts, but this can be reverted. I'll run some benchmarks to arrive at an acceptable tradeoff.

Related owncloud/ocis#2525