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 (Intentionally not setting the variable at this time, so we can decide when to make the change.)
• 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