Fix alignment for 'Add new server' button

[yuya-oc]

Before submitting, please confirm you've

• read and understood our Contributing Guidelines
• completed Mattermost Contributor Agreement
• executed npm run lint:js for proper code formatting

Please provide the following information:

Summary
Fix alignment for 'Add new server' button. (+)

v3.7.0 on Mac:

PR on Mac:

It looks Button component makes the alignment difficult. So I stopped using Button. And '+' is a plain text font, so I think it's difficult to layout at "complete center".
Please see the actual alignment.

Issue link
#540 (comment)

Test Cases

1. Add two servers at least.
2. Open the main page.
3. See tab bar.
4. The '+' button should work as expected.

Additional Notes
https://circleci.com/gh/yuya-oc/desktop/264#artifacts

[jasonblais]

Thanks @yuya-oc!

It's slightly off-center for me on Windows 10 Home

EDIT:

And '+' is a plain text font, so I think it's difficult to layout at "complete center".

It's okay if it's not completely centered, but it would be good to move it a couple of mm down on Windows. It won't be as noticeable anymore at that point.

[yuya-oc]

Not necessary. Screenshots I posted are the result.

I think the font itself for '+' is off-center in English lettering. For example, '+' is slightly off-center on pre-release server (Chrome, Windows 10).

[jasonblais]

The "+" sign next to the tab bar is noticeably off-center though -- it's almost centered on the channel sidebar

[yuya-oc]

https://circleci.com/gh/yuya-oc/desktop/265#artifacts

I used the glyphicon of bootstrap. It's an icon, so probably it would be center-aligned. How about this?

[jasonblais]

Looks great, thanks @yuya-oc!

[yuya-oc]

Thanks for feedback!