[Accessibility]: Multiple H1 tags on Repository Page

[gitstart-sourcegraph]

Problem statement

There are multiple <H1/> elements: the Sourcegraph logo in the navigation bar, and the page title, which produces a warning in ARC Toolkit in the following pages::

1. Repository detail page
2. Repository Commits Page
3. Repository Compare Page

Expected Behaviour

The page title should be an <H2/>, but retain the current style.
All other <H2/>s on the page should be downscaled to <H3/>, but retain their current styles.
No warnings are reported under ARC Toolkit's "Structure and Semantics" tests.

Time estimate

• Pull requests with ~450 lines changed should take X hours at maximum. Ping the reviewer in the spec pull request if time-consuming changes are required.
• Split the work into multiple pull requests if the total diff is bigger than 450 lines of code.

[sourcegraph-bot-2]

Heads up @benvenker @lguychard - the "team/search-product" label was applied to this issue.

[gitstart-app]

Here is the GitStart Ticket for this issue: https://app.gitstart.com/clients/sourcegraph/tickets/SG-39291