Improve Gallery (grid view) on Mobile

[jauyong]

Feature description

As a user, I would like an improved experience for the gallery block on a mobile device.

This came from Yasmine's request regarding to the grid view for the Gallery block on mobile. See slack message here.

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Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

1. Conduct quick competitive analysis of similar blocks in the WordPress space to identify the best method for showcasing gallery blocks and giving content creators the best controls to modify the experience. Reviewed by Joshua.
2. The labels do not need to be cut off but break on the word (ie multiple lines) See Add Line Breaks to Gallery Block #497
3. Allow admins to set number of columns per device (desktop, tablet, mobile). The following are the defaults:
   1. desktop = 5 or 6,
   2. tablet (768px) = 3 or 4,
   3. mobile = 1
4. On mobile, I should be able to set 1 column

Implementation brief

QA testing instructions

Demo

Changelog entry

[jwold]

@jauyong I can't remember if there was anything needed from me on this one. I think we're ready for engineering.

[jauyong]

@jwold we were thinking of having default columns for each device type. what are your thoughts?
(desktop = 5 or 6, tablet (768px) = 3 or 4, mobile = 1)

[jauyong]

@jauyong to break this out into a separate ticket:
AC2
The labels do not need to be cut off but break on the word (ie multiple lines)

[jwold]

@jwold we were thinking of having default columns for each device type. what are your thoughts?
(desktop = 5 or 6, tablet (768px) = 3 or 4, mobile = 1)

That works.

[ravichdev]

@csossi This is ready for QA