Decode hmtl entities in placeholders

[jahvi]

Description

This is my attempt at fixing #4933, I'm using decodeEntities to decode special HTML characters that might be present on the post title / content when using custom placeholders using the write_your_story or enter_title_here filters.

How Has This Been Tested?

1. Set new post title and content placeholder using special characters.
2. Checked that the placeholders render the special character itself and not the entity code.

Screenshots (jpeg or gifs if applicable):

Before	After

Types of changes

Fixes #4933

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code has proper inline documentation.

[aduth]

Looks good, thanks. Left one question.

[aduth]

Should we decode entities in the translated string as well, in case the translation includes entities?

Might speak to a broader point of whether we should want to bake this into the behavior of __ automatically.

[jahvi]

Baking it into __ automatically sounds like a good idea, I wonder if there's a case where you wouldn't want this in a translation.

[jahvi]

Looks like #5490 can potentially take care of encoding __ strings, as mentioned in #4933 it might be worth changing this to a filter instead, I can change my PR if that's the best way to go.