APIv4 Explorer - Adjust 'Index' field to fit better on narrow screens

[colemanw]

Overview

Makes the API Explorer header controls fit better on narrow screens.

Before

"Index" field unnecessarily wide, breaks narrow layouts.

After

Takes up a bit less space, better for narrow screens.

Comments

@braders can you OK this one?

[civibot]

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[braders]

@colemanw Aside from the failing test (missing semi-colon earlier in the file), this seems very sensible. There is no need for such a wide index field. The layout will still break at some screensizes, but at fewer screensizes than was the case before.