Add a "Show context" button to comment notifications

[felipeforte]

Currently, there is not option to "show context" in the menu. The three dots have the options "copy link" and "view source"

[dessalines]

Agree, this is very necessary. Lemmy-ui uses a front end hack to do this currently, because the best way to do it is to add a comment endpoint. Its probably one of the last bigger features lemmy is missing.