Apply a maximum server tab width of 17px

[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.
• I have read contributing guidelines.

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Summary
Add a maximum server tab width of 17px

Steps to reproduce

Create a very long server tab name, 40-50 characters long

Expected behavior

Server name is truncated (common in web browsers such as Firefox)

Observed behavior

The entire server name appears in the tab bar. This can cause issues where if you have two or three servers each with a long server name, the last one isn't visible on the desktop app UI

Possible fixes

Add a maximum server tab width of 17px. This should fit up to 20 character server names.

If the name doesn't fit within 17px, the server name should be truncated with ... at the end.

[jasonblais]

^@yuya-oc feel free to comment if you feel the 17px limit is too restrictive, but I estimated it based on the Firefox browser tab widths.

(in Chrome, the browser tab width auto-adjusts based on how many tabs you have open, but I don't think we need that since it's not likely for a user to belong to a lot of Mattermost instances at once)