Press This: Set explicit user agent

[kraftbj]

WordPress sends a generic "WordPress/$ver" string as the user agent for HTTP requests.

When the bookmarklet is used, Press This does not display the URL entry form, so if a parent site is blocking the generic UA string (pingback spam, etc), then PT will load with no content and not a way to attempt another site.

To help users and other entities monitor, filter, etc traffic from PT, a more specific UA needs to be set.

Marking as a bug as it causes PT to be unusable on certain sites (e.g. Medium.com)

[SergeyBiryukov]

In 33061:

Press This: Set explicit user agent.

props kraftbj.
fixes #32864.

[dd32]

In 33062:

Press This: Rewrite WP_Press_This::fetch_source_html() to utilise the HTTP API better without the need for a local temporary file.
This reverts [33061] but keeps it's custom UA string.

See #32864