Radical simplification for 3.0/MSVC 2017

* Switch as much as possible to PowerShell (no more cmd.exe!)
* Most things are installed thru Chocolatey
* VCPkg replaces the ICU, OpenSSL and libcurl builds
* Switch to SpiderMonkey 60
  * SM 60 has a vastly simplified build process now, with only
    one minor code change required!
* Massive WiX installer improvements
  * Prompts for an admin user/password as CouchDB 3.0 requires
    * Will not overwrite existing credentials if in place
  * No longer remove user-modified config files, closing apache/couchdb#1989
    * Also will not overwrite them on install, naturally :)
  * Checkbox to disable installation of the Windows service
  * Silent install support for all the above
  * Friendly link to the online release notes in the exit dialog

.gitignore

@@ -1,4 +1,5 @@
 *.swp
+*.dll
 release/*
 bits/*.gz
 bits/*.exe
@@ -10,11 +11,15 @@ bits/ftp*
 bits/*.iso
 bits/*.7z
 bits/a2session.txt
+installer/CustomAction/obj
+installer/CustomAction/Microsoft.Deployment.WindowsInstaller.xml
 /.project
 *~
+*.swp
 *.wixobj
 *.wixpdb
 *.msi
+*.pdb
 installer/couchdb.wxs
 installer/couchdbfiles.wxs
 build_*.txt