Change 354916: Remove registerStyleModule() plugin API

Use plugin.styleApi().insertCSSRule() instead.

Under Gerrit 3.7 two warnings are emitted to the console:

The deprecated plugin API 'registerStyleModule()' was called with parameters 'app-theme' and 'wm-light-style'.
The deprecated plugin API 'registerStyleModule()' was called with parameters 'app-theme' and 'wm-common-style'.
registerStyleModule

Those are due to our plugins/wm-app-theme.js plugin.

See Gerrit 3.7 deprecation change at https://gerrit-review.googlesource.com/c/gerrit/+/331259 , specifically the doc change at https://gerrit-review.googlesource.com/c/gerrit/+/331259/1/Documentation/pg-plugin-dev.txt

In Gerrit 3.8 there is plugin.styleApi().insertCSSRule() but that is not available in 3.7 ....