Improve markdown-it emoji list column display

[kimsible]

Fix #3245

• Use CSS flex instead of a classic table to extend the columns on the full modal width and make it responsive ;
• Center the columns ;
• Populate the the markdown-it emoji list from object to array to avoid an alphabetical order of the keys.

[rigelk]

@kimsible is this ready for review?

[kimsible]

@kimsible is this ready for review?

I think yes, because the flex percent solution looks to not be adapted to the longest markup names of emojis.

[rigelk]

@kimsible thanks for your quick response to the issue and sensible fix! Also thank you @test2a for your review.