Include version in useragent string, make it readonly

[HebaruSan]

Background

CKAN and NetKAN use a custom useragent string that identifies them uniquely and works well with most web sites (SourceForge's API being a notable exception, see #4172). This can be overridden with the --net-useragent command line option, which we use to mark the Inflator as a bot so it can be excluded from SpaceDock download counts (see #3490).

Motivation

The useragent string format has a place to put a version:

https://deviceatlas.com/blog/list-of-user-agent-strings

Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
                                   ^^^

... but currently we don't do that. It would be nice if server operators could tell from their logs what versions of CKAN people are using to download.

In addition, the --net-useragent handling is not implemented as well as it could be. The static string Net.UserAgentString is effectively treated as a global variable, written and read freely by whatever assorted random code feels like it, which is bad for reliability, threading, tests, etc. Current versions of vscode rightly raise a warning about this.

Changes

• Now Net.UserAgentString is readonly and includes Meta.ReleaseVersion in its default value, so web servers will know which CKAN is being used, and so we can do a future SpaceDock enhancement to track this if we feel like it
• Now the places that used to overwrite Net.UserAgentString instead pass the user's override value through the parameter stack until reaching one of the functions in Net, which are updated to use the override if passed and fall back to Net.UserAgentString otherwise.