Allow plugin theme files to live under the /theme and /local directory

Bug 1231755. In order to make it easier for custom theme authors to customize
the appearance of plugins, I'm adding the /theme directory to the search path
for plugin files.

Example: smarty->display('artefact:fun:path/to/file.tpl')

Old searchpath:

/artefact/fun/theme/(themename)/path/to/file.tpl

New searchpath (in order of precedence):

1. /local/theme/artefact/fun/templates/path/to/file.tpl
2. /theme/(themename)/artefact/fun/templates/path/to/file.tpl
3. /artefact/fun/theme/(themename)/path/to/file.tpl

Further notes:

The /local directory doesn't have separate copies for separate
themes. It's a sitewide override of all themes.

For some reason theme directories under plugins don't have a "templates"
directory. The templates sit loose under the theme directory. In my new
version, plugin directories under themes DO have a "templates" directory.

Change-Id: Id90ee1d1dec0bda93a0d30e54229fd7dacbe5b96
Merge "Allow plugin theme files to live under the /theme and /local directory"