Add site title partial

[jazairi]

Why these changes are being introduced:

The GeoData visual design includes a heading at the top of each
page, along with some descriptive text.

Relevant ticket(s):

https://mitlibraries.atlassian.net/browse/GDT-182

How this addresses that need:

This adds the requested element as a partial, with conditional
copy for GDT versus non-GDT applications. It also moves the label text
to the title attribute, as the label is is now visually redundant but
may still be useful to screen reader users.

Side effects of this change:

• The input label before form submit is now the same as the placeholder
  text. This feels fine since screen readers might not read placeholder
  text, but they do consistently rely on labels.
• The copy for non-GDT titles did not come from UXWS and is subject
  to change.

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?

NO

[coveralls]

Pull Request Test Coverage Report for Build 7980315778

Details

• 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 99.452%

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Totals
Change from base Build 7978620685:	0.0%
Covered Lines:	363
Relevant Lines:	365

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💛 - Coveralls

[JPrevost]

I'm wondering if we should just remove this now:

https://github.com/MITLibraries/timdex-ui/blob/main/app/views/layouts/_site_nav.html.erb#L3-L6

If we have a site title and an optional platform name in the header, the remaining bit in the "nav" seems redundant. The alternative would be something like only displaying this version if PLATFORM_NAME is set but that feels like a side effect and not something we really need to support.

Requested change: hd-2 class on non-Geo site title in the new partial.

The other bits I'm open for change or not depending on how you prefer.

[JPrevost]

We likely want to use the same class for non-GDT. hd-2 feels like a better size than our plain h1.

[jazairi]

Yep, good call. That was an oversight on my part.