Create Web UI maintenance branch for 3.0.2

We need to create a maintenance branch for Web UI for version 3.0.2, to be released for LTS 2021. We also need to:

1. Make the maintenance branches of both Web UI and Elements (ELEMENTS-1180) discoverable on JX
2. Update package so that it only depends on an LTS distribution of the server
3. Update dependencies on elements to use caret instead of tild on master (but keep tild on the maintenance branch)