Handle oversized plugins

[Xpirix]

Fix for #67

The feature already exists but is not triggered correctly because the NGINX client_max_body_size is set 20M and the max size allowed by the validator is 25Mb.
The suggestion here is to keep the validators limit and increase the NGINX client_max_body_size to 30M. In this case, the validator will raise the error explicitly on the upload page when the plugin size limit is reached without being handled by NGINX and generate the Cloudflare error.

Additional changes:

• Update the Publish a plugin docs to include this limit
• Add the docs for the Approval process (ported from the old version)