Warn if neither a Bugzilla issue nor changelog entry is provided

[wilzbach]

This is one of the top 3 items that newcomers tend to forget.

We could have the bot ignore a couple of prefixes e.g. [Trivial], [Style].

So my idea would be to send a comment on all PRs (except for explicitly ignored one) and saying something like:

Hi @awesomedcontributor.
Thanks a lot for your contribution! We have seen that you have neither included a bug reference nor changelog entry.
In case you forgot, please see our [documentation](https://wiki.dlang.org/Contributing_to_Phobos).

Implementation would be simple as it's just the default case:

• no issue found
• title doesn't start with X
• diff doesn't contain files in changelog/

I am just not sure whether this would be useful in general or too spamful due to false positives.

[wilzbach]

This was fixed in #89