Add ImageGrid component and use it for RelatedImages

[obulat]

Fixes

Related #280 by @obulat

Description

This PR extracts the functionality of SearchGridManualLoad that has to do with displaying images and an optional Load more button into a new ImageGrid component.
RelatedImages component is refactored to be responsible for fetching related images, and use ImageGrid to display them. This makes the images part of related Vuex store unnecessary, and also simplifies the Vuex isFetching and isFetchingError states: now they are only changed by Image search and Single image result [which should actually control its state separately].

I think the SearchGrid component will also need to be refactored to use the ImageGrid component later.
The ImageCell is copied as-is from the SearchGridCell to ensure that possible changes later do not introduce unwanted bugs.

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[zackkrida]

Great. There used to be such an irritating bug where if loading related images failed, the site would show related images from the previous image, due to these being held in a global store.

[krysal]

Now these components seem more in line with the "Single-responsibility principle". Awesome!

[sarayourfriend]

I love getting rid of global state 🎉 I'm running into some local dev issues so I can't test it though 😢

[krysal]

LGTM!

[sarayourfriend]

LGTM! 🚀