[UI] Various UI grouping fixes

Various changes:

- [ ] Show a count if there are merged issues \[1\]
- [ ] For merged fingerprints make it obvious that there are links to open \[2\]
- [ ] Make it obvious that we group by the In-app Exception rather than the global exception \[3\]
- [ ] Either don't show the current issue in the Merged Issues tab, or make it clear it's the current issue and don't show it in the count of how many issues have been merged in. (Screenshot 4 is of an issue where no merging has happened and screenshot 2 is an issue with one other issue merged in.) [Here](https://github.com/getsentry/sentry/issues/56334#issuecomment-1733692221) is an instance of a customer being confused by this. \[4\]
- [X] Kill Grouping tab as it's deprecated \[5\]
- [ ] Make activity tab entries for grouping clearer \[6\]\[7\]

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