#39 add item user references

[phoenixbox]

What's this PR do?

Updates the mentions within the description of the add-item modal to include the syntax whcih the api expects:

@[Shane Rogers](123)

Changed to:

@[shane Rogers](pk:123)

Also adds a max-height to the mentions suggestions so that the modal doesn't overflow

Where should the reviewer start?

component/helpers.js

How should this be manually tested?

Create an item in Kanban using a mention in the description Check the generated tickets description Ensure that the mention links redirect to that members activity page

What are the relevant tickets?

#39

Screenshots (if appropriate)