Initial JetBrains support - Idea and PyCharm Community Editions.

Work in progress, do not merge yet.

DownloadCenter grew support for custom headers in DownloadItems.
Support for PyCharm and Idea Community Editions.

Need to add tests and refactor the JetBrains stuff a little so more logic is in the base JetBrains installer - this will allow us to add support for more IDEs easily.
Yes. There was an issue with this test that I thought was the timeout but it was something different, and I forgot to revert the value. Idea does take a long time to download for some reason.
Will revert.