[TheBell] Add new bridge for TheBell (#4954)* [TheBell] Add new extractor* [TheBell] Detect bridge by URL* [TheBell] Support categories* [TheBell] Code cleanup* [TheBell] Handle relative image URL's in articles* [TheBell] Use default cache timeout* [TheBell] Fix code style warnings* [TheBell] List available news categories* [TheBell] List available news categories* [TheBell] Add selected category to feed name* [TheBell] Set default number of articles in parameters* [TheBell] Set default number of articles in parametersRemove required setting* [TheBell] Add investigations category