Add a drop-down selection of built-in User-Agent strings in the /network/http form

[postmodern]

The /network/http form should provide a drop-down selection list of built-in User-Agent strings from Ronin::Support::Network::HTTP::UserAgents, which should then be mapped to a Symbol value by Validations::HTTPParams. It should also provide a text field for entering a custom User-Agent string value.