Resolves #281 reworks `get_length_info()` and its uses

[yuliu]

Resolves #281. To test this PR, one needs to have https://github.com/mybb/mybb/pull/4497 on their MyBB.

• Adds a function get_column_length_info() that returns extended information (incl. int, bin, etc. types) to replace get_length_info().
• Replace uses of get_length_info() in Converter_Module->prepare_insert_array() and Converter->check_column_length() with get_column_length_info().
• Adds another logging funciton Converter->log_column_error_notice_in_progress() to save and log in-process warnings.
• Changes report output content (text and HTML) to include in-process warnings.
• Some minor tweaks to prevent the global $import_session content being too long on saving to mybb_datacache.
• Fixes a typo in get_length_info().
• Deprecates get_length_info().