Refactor posting and make character limit configurable.

config/config.exs

lib/pleroma/web/common_api/common_api.ex

lib/pleroma/web/common_api/utils.ex

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

lib/pleroma/web/ostatus/handlers/note_handler.ex

lib/pleroma/web/twitter_api/controllers/util_controller.ex

...roma/web/twitter_api/representers/activity_representer.ex

lib/pleroma/web/twitter_api/twitter_api.ex

lib/pleroma/web/twitter_api/views/user_view.ex

test/web/common_api/common_api_utils_test.exs

test/web/twitter_api/twitter_api_test.exs

test/web/twitter_api/views/user_view_test.exs