Standardize PKP PN naming

[cjwetherington]

Based on this 2017 PKP announcement,

the network previously known as the PKP Private LOCKSS Network (PKP PLN) is now the PKP Preservation Network (PKP PN)

Standardizing various inconsistent references to the network in the plugin's localization files:

• PLN -> PN
• PKP-PN -> PKP PN
• PKP|PN -> PKP PN
• Private LOCKSS Network -> Preservation Network (English appearances)
• Conservation Network -> Preservation Network (apparent mis-translation)
• Rede Privada LOCKSS -> Rede de Preservação (Portuguese appearances)

Of note are the various references to "PKP-PLN" in the plugin's Terms of Use and Status sections, which seemingly come from the network itself, which is what led me to starting this work. It would be great to bring those into alignment as well, as the inconsistencies can be somewhat confusing.

[cjwetherington]

Thanks, @jonasraoni