Plugin Upload Handler improvements

[Ipstenu]

Based on the last 6 months of plugin submissions, we need to improve the language used.

• Many translation changes to improve explanations as to why resubmitting plugins isn't needed.
• Updating links to the handbook vs the codex.
• Explain what a URI is and that they need to be UNIQUE
• Explain you don't need both a plugin URI and an author URI
• Explain we're missing the README file.
• Warn that plugin slugs are subject to change (again)
• Rephrasing parts of the submission email to be more clear about where they are in the process.

Also there's a new feature to check if your plugin slug STARTS with a probably bad term. This has been tested locally but really could use more eyes.

[SergeyBiryukov]

In 8470:

Plugin Directory: Upload: Improve the language used in explanations for upload errors.

Add a check whether the uploaded plugin starts with a commonly abused slug.

Props Ipstenu.
Fixes #4293.

[SergeyBiryukov]

In 8471:

Plugin Directory: Upload: Fix typo in the warning for reserved slugs.

Props tobifjellner.
See #4293.

[SergeyBiryukov]

In 8472:

Plugin Directory: Upload: Correct explanation of the URI acronym.

Props tobifjellner.
See #4293.