feat(web): simplify header navigation (#1427)

* removes the tabbed navigation completely
* makes it possible to see the custom name of the repository


tabs (and really all not "main" nav) were not in use and the features
may not work the same way in the future, so rather than redesigning, the
whole tab bare was removed.

for the custom name, if it's `__default`, the the default `git-bug` text
is displayed; otherwise, the name of the repo.