Add support for gerrit 3.4+

[twmr]

assignees no longer exit in recent gerrit release, instead attention sets can now be used (
https://gerrit-review.googlesource.com/Documentation/user-attention-set.html)

• Add support for displaying reviewers in the dashboard
• Add support for displaying the attention set in the dashboard
• Add support for changing the attention set in the dashboard

[twmr]

In a first step we can add support for just displaying the reviewers in the dashboard