Changes in get_discussions_by_author_before_date and other small fixes

Added author validation and removed overlap from paging in condenser_api.get_discussions_by_author_before_date (note that there was no similar functionality in bridge to reuse).

Added validation of truncate_body parameter to every call that has it. Also that parameter was added to get_discussions_by_author_before_date for consistency.

Unified tag validation and fixed null limit problem in condenser_api.get_discussions_by_blog.