Improve milestone handling

1) Perhaps add an option to not show the warning when we target a hotfix, perhaps on the app side, or Dangermattic can provide a helper for that.
E.g. Dangerfile to make an exception for when PR targets /release\/\d+\.\d+\.\d+/

2) Handle empty `to_date`.
Now, we get this error when the milestone has no due date:

```console
Dangerfile:55:in `eval_file': (Danger::DSLError)
[!] Invalid `Dangerfile` file: undefined method `to_date' for nil:NilClass
# from Dangerfile:55
# -------------------------------------------
# if milestone
> code_freeze_date = milestone['due_on'].to_date
# submission_date = code_freeze_date + 7
# -------------------------------------------
from /usr/local/bundle/gems/danger-9.4.3/lib/danger/danger_core/dangerfile.rb:311:in `eval'
```

Start at Dangerfile line 55, where milestone['due_on'] is converted with to_date, and inspect how the target branch or release pattern is available. Check the existing milestone warning flow and the referenced Dangerfile conditions. Done means hotfix targets can avoid the warning and milestones without a due date no longer raise the shown error.

Written by the indexing model from the issue text.