Update rst icons once Learning Activities icons are in KDS

[nucleogenesis]

When learningequality/kolibri-design-system#232 is completed, the rstIconReplacements file from KDS (https://github.com/learningequality/kolibri-design-system/blob/v0.2.x/docs/rstIconReplacements.txt) should be copy-pasted into the file of the same name in this repository.

[indirectlylit]

ref: https://design-system.learningequality.org/icons/#rst