Add `app-change delete` command which allows easy decluttering of stored app changes

**Describe the problem/challenge you have**
We do not have a command allowing easy decluttering of app-changes through `kapp` as of today.

**Describe the solution you'd like**
- The `app-change delete` command could have the same flags to filter through changes as those for `app-change list` mentioned in #339.
- Alternatively (or additionally) we could have a `--max-app-changes` flag which which keeps n changes while deleting the rest.
- If we have both these options there should be some ground rules deciding how they interact with each other

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