tweak/mention_menu

You are the most delightfull GP / multimedia nerd I know adam <3

The notes menu now lets you select users to @mention in a post, so you don't have to copy/paste usernames or lookup mad spelling.

Good, now I can tag you without looking up how many q there are in Adam

After some fiddling and extensive trial and error (aka, forgetting to check the usernames before clicking @mentions) I deem this feature successful.
Great job as always.