Change Mattermost server tab bar to be a similar height to Chrome tab bar

[jasonblais]

I confirm (by marking "x" in the [ ] below):

• This is not a troubleshooting question. Troubleshooting questions go here: http://www.mattermost.org/troubleshoot/.
• This doesn't reproduce on web browsers (such as in Chrome). If it does, issue reports go to the Mattermost Server repository.
• [ X] I have read contributing guidelines.

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Summary
Change Mattermost server tab bar to be a similar height to Chrome tab bar. Currently the tabs are significantly larger compared to browser tabs.

Moreover, the current height is not an integer in physical dots on the monitor, causing slight blurriness on the app.

See further details here: #459 (comment)

[jasonblais]

Resolved by #489