Limit check run fields size

[EnricoMi]

Github API limits the size of some API call JSON fields, especially when creating check runs:
https://developer.github.com/v3/checks/runs/#annotations-object-1

Annotation:

• message: 64 KB
• title: 255 characters
• raw_details: 64 KB

Limit these fields by abbreviating the strings with ellipsis in the middle of the string. Make sure the title cannot get larger than 255 characters without abbreviating it.

Limiting a string by bytes is tricky as some characters may use multiple bytes. Easiest would be to limit it to 16k characters, which should be safe in terms of bytes in any situation.