Change add_context_info method in typos

[irinakhismatullina]

In utils.add_context_info() save context parts as strings (space-separated-tokens) , not lists.
Example: instead of ["get", "token"] use "get token".

Simplifies storing datasets in .csv files.

Based on #626.

[vmarkovtsev]

@irinakhismatullina Now we need to rebase here

[m09]

I didn't review the semantic changes themselves since it's out of context in this PR. More generally, the files contain many wrong type annotations where an Optional is lacking. When a default value is None, an Optional[] is needed.