Move import of mixins to embedded layouts

[krysal]

Description

As mentioned in the title, this PR moves the imports of mixins of each page to the embedded and embedded-with-nav-search layouts, as they're oriented to handle aspects related to the embedded version of Openverse in wp.org. This will save us some lines of code repetition and will be easier to apply changes to corresponding pages in the future, if required.

Tests

Repeating the same steps for Sync locale... #204 and check that it works the same way.

Checklist

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[krysal]

I added a small fix to center the input form on the main search page. In English, it was not noticeable that it was not aligned to the center but in other languages it's more obvious.

Before

After

[dhruvkb]

It was a great idea to move the code to layouts and the implementation is equally great.