Previously plugins could declare js_dependencies and css_dependencies
which in theory would then be embedded in the page by themes.
Practically this does not make sense when you have multiple themes
since the JS & CSS needs to be written with a specific theme in mind
in order to support it.
Which is also why js_dependencies and css_dependencies was actually only
used by the Oscar theme and ignored by the Simple theme because the
Simple theme doesn't bundle jQuery and has a slightly different page
markup.
This commit removes this broken feature and also moves the *.js files in
searx/static/plugins/js/ to searx/static/themes/oscar/src/js/plugins
(where they actually belong).
The oscar-specific CSS is now simply injected via the oscar-specific JS.
Fixes#769.
