Fix zoom button positioning and size in styledMode

[cfarm]

Fixes wonky zoom buttons that appeared after turning on styledMode for geographic maps in #171

Additions

• larger font-size for more legible zoom-out button icon (minus sign)
• custom positioning of the icon elements with CSS

Testing

• gulp build && gulp watch

Screenshots

Before

After

Notes

• I tested this in Chrome, Firefox, IE, Edge, and mobile browsers. it is not pixel-perfect across browsers but I optimized for Chrome and IE/Edge. The minus sign appears to be 1 pixel lower or so in Firefox:
  

Todos

Checklist

• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows the standards laid out in the front end playbook
• Passes all existing automated tests
• New functions include new tests
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged
• Visually tested in supported browsers and devices
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)

[cfarm]

@Scotchester this is the best I could do for IE11:

• Removed the larger font-size that I had added to make the minus sign more prominent. This was making it positioned at the bottom of the button which was the majority of the wonkiness
• I tried using Highcharts options to set the transform attribute, but those options actually only allow me to set the x and y attributes for the button as a whole, not just the plus and minus text. So I had to stick with CSS to position them across browsers that support it.

IE11

Chrome

Firefox

[Scotchester]

Looks good enough to me, @cfarm.