Bug fixes and minor features

Server

b568849 add CORS headers to S3 media handler
2da517f more S3 config options per #673
ba60917 use types.TimeNow() instead of time.Now() in serveStatus
91cbf06 Change status output to json.
2aba8dd lint warning: required comment added

Tindroid

Bug fixes: https://github.com/tinode/tindroid/releases/tag/v0.17.6

Webapp

Bug fixes: https://github.com/tinode/webapp/releases/tag/v0.17.6

Javascript SDK

Bug fixes: https://github.com/tinode/tinode-js/releases/tag/v0.17.6