After choosing an @-mention option, backspacing should reopen autocomplete #1967
Description
In Web, if an @-mention autocomplete option is chosen, pressing backspace will reopen the autocomplete. For example, if the chosen option puts @**Chris Bobbe** in the compose box and then backspace is pressed until it becomes something like @**Chris, the autocomplete will be shown for all the matching results of Chris, which means ignoring the starting ** from the query. We implemented a similar behavior for the channel link autocomplete in #1902, but this issue is for doing so in @-mention autocomplete.