Shared UI - Header and Footer

[CallumNZ]

Reference: https://github.com/GeoNet/tickets/issues/11449 , https://github.com/GeoNet/tickets/issues/11435

Changes proposed in this pull request:

• Upgrade go version to 1.16
• Add UI package, includes functions that return HTML for GeoNet header, footer, and basic header.
• Add hidden Mac file to .gitignore
• Update CSP so that search form in header works in any GeoNet domain.

Production Changes

The following production changes are required to deploy these changes:

• None

Review

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Code Review Guide

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[bpeng]

Code looks good, and handy to use