Fix the detection of Dynalist tags

[tanius]

Currently, @mentions and #name tags for task assignment are detected here with a regex \s@([a-z]{3,15}) and \s#([a-z]{3,15}), respectively.

However, some usernames on edgeryders.eu and thus tag names in our Dynalist documents contains special characters, for example: #anique.yael. It's a valid tag in Dynalist, as can be seen from the gray background formatting for the whole thing (not stopping before the dot) when entering it in Dynalist. Likewise, the restriction to 3-15 characters in length is arbitrary.

The clean solution here will be to detect all tags that are valid in Dynalist. For that, find out first what special characters can be contained in Dynalist tags, and adapt the regex accordingly. We should not try to apply any stricter rules on "what is a tag" than Dynalist, as others will use our software without these same conventions (that might come from Discourse usernames in our case, but others won't have Discourse).

(Then, as it is already happening, the application will try to look up an associated e-mail for each tag appearance, and send an e-mail for those where it succeeds. No changes needed in that part.)