Add accessible names to pipeline schedules buttons

What does this MR do and why?

Adds accessible names to the buttons on the pipeline schedules page.

Why? To make the page more accessible and to reduce console warning noise.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. git checkout master
2. navigate to a project
3. in the sidebar, choose Build => Pipeline schedules
4. in the devtools console, many warnings like this are shown: [gl-button]: Accessible name missing. Please add inner text or aria-label.
5. git checkout add-accessible-names-to-pipeline-schedules-buttons
6. warnings should no longer be shown

MR acceptance checklist

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