Fix NavBar main button styles

Created by: oleggromov

Problem

We recently discovered a regression after migrating from Dropdown to MenuItem.

https://user-images.githubusercontent.com/19534377/154088419-ce214fbc-e46f-472b-a21e-f7f4e2d4504a.mov

There're 2 issues with the current state:

1. Dropdown button styles are different - there's a transition, an unwanted border radius, and bolder font weight
2. Focus behavior is different too - it's shown on the entire panel instead of individual menu items

Solution

We currently fix only the 1st issue because the 2nd one is deeply ingrained into the MenuList -> Popover (PopoverContent) -> FloatingPanel rendering tree, and it seems to be a sophisticated change with a questionable outcome. The current implementation is just a bit different, not wrong.

Before

At the dfd241cc5ddf0acc080565239ad674f81b8af427 commit:

https://user-images.githubusercontent.com/2196347/154300735-ddeac910-d9d0-4e7b-8b58-765b71507bc5.mov

After

https://user-images.githubusercontent.com/2196347/154302587-27d51386-a405-4c72-8c1d-b6271b1f1655.mov

Test Plan

1. Run Sourcegraph locally
2. check that the styles are fine