Change check.gif to an actual gif (was a PNG)

[colemanw]

Overview

Fixes https://lab.civicrm.org/dev/core/issues/1503
An image file named check.gif was actually a PNG image. I've re-saved it as a real gif.

Before

Mismatched file extension.

After

File extension and file type match.

Comments

This slightly changes the image as GIFs don't support 8-bit alpha masks. Oh well, IMO image-based icons are deprecated anyway.

[civibot]

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