Make plugin install deep-linking easier

[rmccue]

Deeplinking into the WordPress admin is generally a better user experience than pointing users to the plugin repository, if you already know their admin URL. With autodiscovery in the REST API, this becomes even easier, as you can find their admin URL from just their domain (or site address).

As an example, the ​Stream plugin homepage does this (minus the REST API discovery) to great effect.

However, there are numerous problems with this:

• The only screen to really point users at is the Search, as plugin-information is intended for an iframe and doesn't include the admin "chrome"
• Since it's a search, other results might turn up, or your desired plugin may not appear at all if the plugin isn't marked as compatible (i.e. try the Stream install on trunk)
• Search parameters don't follow redirects (e.g. redirect to Network Admin)

We should make this easier. Additionally, WordPress.org could add an install helper to do this for every plugin on the repository.

Adding either a REST API endpoint which simply redirects (still requiring user interaction), or an admin action redirect would be great. I think the latter is probably better, and I'd like to propose something like /wp-admin/?action=install-plugin&slug=wp-stream as a redirect.

[dd32]

Mostly a duplicate of #18289 (which was near-impossible to find)