Project-specific themes were only implemented in the GWT UI, which has
been removed.
Remove the ThemeInfo entity, and no longer include the 'theme' field in
the ConfigInfo entity returned by the project config REST API endpoint.
This is a breaking change for any client that still uses that endpoint.
The custom site header HTML, footer HTML, and CSS files are still used
by login screens and the internally managed Gitweb server. Update the
theme documentation to mention this.
PolyGerrit has site-wide theme support, but it is not documented in the
existing config-themes.txt, and adding it is not within the scope of
this change.
Change-Id: Ibe824a9416fac65bd365407b8ce22e03515b5b52
3fbdf98c9a