PLANET-5481 Image Archive UI improvements

[planet-4]

We integrated the GPI Image Archive into the master-theme at greenpeace/planet4-master-theme#1135. It's currently "masked" under a Feature toggle (Planet 4 > Features).

There are some low-hanging fruits we can pick up to improve UI, listed below.

Tasks

• Use a dashicon (eg. format-image)
• Push the menu right after the Media (maybe remove "GPI" to make it shorter)
• Rename the first submenu item to "Library"
• Use same UI elements for the search bar and the buttons as the rest of wp-admin
• Use grid instead of flexbox for displaying the image, to have them use the same dimensions (similar to the current medialibrary plugin)
• Image link after importing should open on the same window

Reporter: nroussos
Sections: Media Library

Potentially affected repositories: planet4-master-theme

[comzeradd]

Done in https://github.com/greenpeace/planet4-master-theme/pull/1215