Plugin Directory: Check for license on upload

[Ipstenu]

Related to #1944

In order to enforce licenses, the first step is to make sure everyone declares one on upload. This will also catch some people who make a junk readme.

[Otto42]

Question: if we're going to start adding this sort of thing, then would it be worth reviving ​https://wordpress.org/plugins/plugin-check/ and using that, similar to how we use theme check now?

[Ipstenu]

@Otto42 Probably. I'll see if I can port it over now that I have a better idea of how to check for everything via bash.

[Otto42]

Well, it's not difficult to do, mainly I was just wondering if there were any other checks we have that we might make modular and move into a plugin form like this, for much the same reason I made theme check initially.

[dd32]

In 9648:

Plugin Directory: Require a license be specified in the readme.txt.

Props Ipstenu.
Fixes #4719.
See #1944.

[Otto42]

In 9655:

Plugin Directory: Reverse the logic on the license check introduced in [9648]. Fixes #4719.

[casiepa]

Related: #5126