Fixes#4558.
This makes a number of fixes to get the context help working correctly
in Keyman 14:
1. Adds support for `redirect:` header in help .md files so that we can
do redirects for the context content in Keyman for Windows help in the
lua filters.
2. Renames Keyman for Windows help files to .htm, because that's what
the .chm format is expecting, and what Delphi assumes for its .chm
integration, and thus splits the html and htm lua filters.
3. Renames and makes more consistent the various context help pages.
4. Adds redirects for all pages that have sufficient content on other
pages.
5. Fixes the instantiation of help in various Keyman for Windows forms.
6. Defaults to opening help from the Keyman source repo if the calling
app is running in the source repo (this is just a development-side
tweak that simplifies my life).
