Miscellaneous Kolibri UI fixes for 0.18

[marcellamaki]

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UI improvements

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1. Fix unnecessary horizontal scroll on users table - Jacob
2. Fix padding on "Create new lesson" page - Misha
3. Update spacing between navbar and container on the "learner's" subpage in coach Alex
4. Adjust padding for consistency on resource preview subpage Liana
5. Fix category icons in search modal - Marcella
6. Remove icon next to report visibility dropdown menu - Richard
7. Fix incorrect string reference for "work skills" on search chips in lesson search results Allan
8. Add additional string for "choose questions manually" UI Alex
9. Add a bit of extra margin around resource titles - Liana
10. Remove the class name from the app bar when it is included in the page - Allan
11. Options button should be a dropdown Allan
12. Remove individual recepients from main page filter Allan
13. Add white drag and drop background to resources in the resources selected page Allan
14. Manual question selection banner does not need to be sticky