Support double hashtags (and double mentions) in newpostmsg

See https://github.com/miguelfreitas/twister-html/commit/706f17869bcb59200b9247df187a54dd52990720 (https://github.com/miguelfreitas/twister-html/pull/202) and https://github.com/miguelfreitas/twister-core/commit/07edcd08a9e5c43c6442b9c8d65cf9b44b2cfd9e (https://github.com/miguelfreitas/twister-core/pull/295)