Gallery not centered in many themes due to margin/padding on UL

[dreamwhisper]

Issue Overview

ULs were used for gallery markup in #3441. However, many themes style lists within the content and add padding or margin to the left side. Thus, galleries appear shifted to the side/uncentered.

Steps to Reproduce (for bugs)

1. Insert a gallery block
2. Activate a theme that adds margin and/or padding to content ULs (Twenty Fourteen is an example with alignnone selected for the block.)
3. Note galleries will be shifted to the side.

Expected Behavior

Galleries would not be shifted by CSS intended for text-based lists.

Current Behavior

Margin and/or padding is adopted from text-based list styling and galleries are unintentionally shifted.

Possible Solution

Reset left/right margin/padding for:

.wp-block-gallery,
.wp-block-gallery.aligncenter,
.wp-block-gallery.alignleft,
.wp-block-gallery.alignnone,
.wp-block-gallery.alignright 

Screenshots / Video

Related Issues and/or PRs

#3441

Todos

• Tests
• Documentation

[ErikXXon]

I also get this problem in my theme.