Hide dropdown on single option

[Kevan-Y]

Fixes

Fixes #312 by @sarayourfriend

Description

When there is only one format option, the dropdown addon is not rendered.

Testing Instructions

1. Empty array alt_files of one audio item in src/data/sample-audio-responses.json
2. Search for that audio
3. Click to show audio detail page

Checklist

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• I added or updated tests for the changes I made (if applicable).
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• I tried running the project locally and verified that there are no visible errors.

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[sarayourfriend]

This is definitely the simplest solution! Thank you!

[zackkrida]

This works great! Once we have a proper Button.vue component, I think it'd be better to remove this logic and conditionally render one or the other. For now this is great.

<DropdownButton v-if="formats.length > 1" />
<StandardButton v-else>Download...</StandardButton>