Fix Message Parsing

[HerHde]

Originally posted by @HerHde in #12 (comment)

Current limitations of the message parsing:

• another parser (showdown) for emojis is used, thus the results may differ from reactions and custom emojis are not translated, yet This is fixed by Some emojis in message texts are not translated into Unicode representation #20
• if mentioning @all, @here and individuals in one message, the latter matches are ignored
• Showdown parses differently than Element (and probably also different from RocketChat)
• No check for mentioned user's existence is performed