Add share popup with GA tracking

Adds ability to share using email, linkedin or directly. Events are
tracked in Google Analytics.

https://www.pivotaltracker.com/story/show/74604946

[Delivers #74604946]
Change Google Analytics property ID

We've separated the pilot planner app as a new property, distinct from
www.sealevelresearch.com. Here's the new property ID.
Increase the header size of the feedback modal
Merge pull request #14 from sealevelresearch/share-button

Add share popup with GA tracking
Merge pull request #15 from sealevelresearch/change-google-analytics-property

Change Google Analytics property ID
Merge pull request #16 from sealevelresearch/increase-feedback-header-size

Increase the header size of the feedback modal
Use GMT not BST

To be consistent, and it's what they're accustomed to.

https://www.pivotaltracker.com/story/show/74938664

[Delivers #74938664]
Merge pull request #17 from sealevelresearch/use-gmt-not-bst

Use GMT not BST