List the letter-sound correspondences where a letter is used

[jo-elimu]

We want to make it easier for content creators to see how letters have been linked to sounds.

At /content/letter/edit/<ID>, display a list of letter-sound correspondences where a letter is used. Place the list in the <aside>.

This is very similar to what was already implemented in https://github.com/elimu-ai/webapp/pull/1519:

See sample at http://hin.elimu.ai/content/sound/edit/14

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Similar to https://github.com/elimu-ai/webapp/issues/1518