Wp-calypso: Saving "Writing" settings in Calypso removes slashes from custom date format

[liviopv]

Steps to reproduce

1. Starting at WP Admin > Settings > General, set up a custom date format with slashes like the default for pt-br j \d\e F \d\e Y
2. Go to My Site > Manage > Settings > Writing in Calypso, make any changes and save
3. The custom date format on WP Admin > Settings > General will appear without slashes, which will cause to display incorrectly on the site

What I expected

Changing the Writing settings should not affect the Date Format

What happened instead

Changing the Writing settings broke the Date Format

Browser / OS version

Any browser or OS

Screenshot / Video

Video: https://cld.wthms.co/vK0jHm

Context / Source

We had a lot of issues with this in the Portuguese Support queues but the volume slowed down to the point we thought the issue was fixed, so we never reported it.

The issue surfaced again and @yoavf recommended reporting it here.

[akirk]

This is not i18n specific, for example if you want to have a date "6th of May, 2019", you need to put jS \o\f M, Y and have the same problem in English.

[carinapilar]

Got another one on 2012619-zen.

Date shows like 5 05America/Sao_Paulo maio 05America/Sao_Paulo 2019.

[liviopv]

2110386-zen

[liviopv]

Another #2151180-zen

[lancewillett]

@donpark Could you please take a look at this one?

[donpark]

@lancewillett sure thing, Lance.

[donpark]

PR fixing this bug is ready for review.

[donpark]

This bug should now be fixed in production. Good job everyone!