Specific rules for openHAB Web UI maintainers

[ghys]

Besides the global rules for maintainers, we have the following additional agreements for openHAB 2 Web UIs:

• Due to the varying technologies and skillsets of the @openhab/webui-maintainers team, and until a common set of technologies for future UIs is agreed upon, as a (temporary?) exception to the general rule individual code owners of a particular Web UI are allowed to merge their own PRs
• Bug fixes and new features for a particular Web UI can be merged by any maintainer who is not a code owner of that Web UI, as long as at least one of the involved code owners gives approval
• New contributions not involving any existing UI, and contributions involving multiple UIs should be reviewed by 2 maintainers, who have code ownership of the affected UIs if applicable.