add first header bar, fix #169

TODO:
* [x] show correctly when multiple people are in cc
* [x] show the toEmail and ccEmail correctly in the title (so it’s visible on hover
* [ ] filter if it’s your email address and show »you« instead of your name
* [x] show »and 4 more …« if the recipients list grows too long, or just ellipsize for now

ref #169