Enhancement: Add handles to easily message mapper

[wireguy]

Similar to #3146 , add handles to the comment editboxes

Example mockup adds 3 users below editbox:

[wireguy]

Would also be nice to have it in the Task Status completion, since 'task history' is not displayed when validating multiple tasks.

[willemarcel]

It's a nice suggestion! I think a better solution would be to automatically list all the users that collaborated with the task, when we type @. What do you think, @wireguy ?

[wireguy]

If you mean to only show users that have collaborated, then no.
I may want to add someone new to the task discussion, as in, 'wille is this a bug' or 'bob can you validate this task'.

There is also a growing list of things to know to type '@' for email, and markup notation.
Would be nice to have a help popup (?) with all of the things you can type or do in the editbox.

[willemarcel]

If you mean to only show users that have collaborated, then no.

Right now, when you type only @, nothing happens, you need to type a letter to start searching users to mention. So we would list the users that interacted with the task while we have only the @ typed. After typing a letter, it would work as it is now.

[wireguy]

so '@' is like an mru list. i like it.

[wireguy]

@willemarcel I thought about this some more, and have an alternate (probably easier to implement)
Instead of the users name being a link to their profile, have it popup a 3 item menu:

• link to their profile https://tasks.hotosm.org/users/wireguy
• copy their username in markdown format @[wireguy] (then I can paste it in the comment editbox)
• link to OSM message (opens in new tab) https://www.openstreetmap.org/message/new/wireguy

if you don't like the menu, then you could keep the link, and add 2 small graphic buttons after
maybe

and