[fix] remove js_dependencies & css_dependencies

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.

docs/dev/plugins.rst

@@ -19,9 +19,6 @@ Example plugin
    description = 'This plugin extends the suggestions with the word "example"'
    default_on = False  # disabled by default
 
-   js_dependencies = tuple()  # optional, list of static js files
-   css_dependencies = tuple()  # optional, list of static css files
-
 
    # attach callback to the post search hook
    #  request: flask request object