In order for frontend to display appropriate messages on why
a suggestion can't be applied, it needs to know the reason.

A new `inapplicable_reason` property is introduced and included in
the API response to achieve that. It is based on the checks to
determine whether a suggestion can be applied.

The following reasons are:

- "You don't have write access to the source branch." - when user
  has no write access to branch.
- "This merge request was merged. To apply this suggestion, edit
  this file directly." - when MR was merged.
- "This merge request is closed. To apply this suggestion, edit
  this file directly." - when MR is closed.
- "Can't apply as the source branch was deleted." - when source
  branch is deleted.
- "Can't apply as this line was changed in a more recent version."
  - when the line where suggestion is on is outdated.
- "Can't apply as these lines were changed in a more recent
  version." - when lines where suggestion is on are outdated.
- "This suggestion already matches its content." - when suggestion
  has the same content as the line it is on.
- "Can't apply this suggestion." - default message when suggestion
  cannot be applied but doesn't fall into any of criteria above.
  We won't show this message anywhere right now but it makes sense
  to return something (e.g. when suggestion is already applied).