Don't use regex replacement to set force_default to true for images under Settings > Discussion > Default Avatar

[henry.wright]

Now that get_avatar() accepts a 5th param, we should use it to set force_default instead of using preg_replace(). Patch to follow...

[henry.wright]

I should mention this will help fix BuddyPress ticket 6298.

Ref: ​https://buddypress.trac.wordpress.org/ticket/6298.

[swissspidy]

Note: The resulting HTML, which is being filtered in default_avatar_select, changes slightly using this patch. Instead of appending forcedefault=1 to the URL, it contains &f=y.

Not a big deal, but I'm not sure if it's big enough to skip for 4.5

[henry.wright]

According to Grav, either f or forcedefault can be used as the param:

​https://en.gravatar.com/site/implement/images/#force-default

[swissspidy]

In 37227:

Admin: Fix default avatar handling in Settings -> Discussion.

By passing the force_default argument to get_avatar() instead of using preg_replace, we make it easier for developers to override the output.

Props henry.wright.
Fixes #34744.