Update theme gem and document PLATFORM_NAME variable

[jazairi]

Why these changes are being introduced:

The GDT project added a feature to the theme gem to conditionally include the platform name in the header if the PLATFORM_NAME environment variable is supplied.

Relevant ticket(s):

https://mitlibraries.atlassian.net/browse/GDT-124

How this addresses that need:

This updates the theme gem to the v1.2 tag, which includes the new feature, and documents the PLATFORM_NAME variable.

Side effects of this change:

None. The app will continue to use the standard header if PLATFORM_NAME is not supplied.

Developer

• All new ENV is documented in README
• All new ENV has been added to Heroku Pipeline, Staging and Prod
• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• There are appropriate tests covering any new functionality
• The documentation has been updated or is unnecessary
• The changes have been verified
• New dependencies are appropriate or there were no changes

Requires database migrations?

NO

Includes new or updated dependencies?

YES

[coveralls]

Pull Request Test Coverage Report for Build 7845471351

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.444%

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Totals
Change from base Build 7835420707:	0.0%
Covered Lines:	358
Relevant Lines:	360

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💛 - Coveralls

[jazairi]

Note to reviewer that the TIMDEX UI title in the local nav will be removed in #113.

[JPrevost]

The platform name seems slightly misaligned to me in the smallest viewports, but that is a theme gem thing that we can loop back to in the future.

edit (also, it just looks misaligned to me. I don't think it actually is which is of course more fun)