Prefill textview with "@" prefix

[eshwartm]

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.

Description

This bug happens when we try to prefill the textview with an "@" prefix and try to edit it or add more prefixes to it. When we go to add another "@" prefix and then select a suggestion, the selected suggestion text attaches itself to the start of the textview.

Reproducible in:

• [x] This is reproducible in the sample project.

SlackTextViewController version: iOS version(s): 9.3 Device(s): Simulator

Steps to reproduce:

1. Open workspace to edit the Messenger-Swift project.
2. Add textViewDidBeginEditing method.
3. Add textView.text = "@Someone " to the method.
4. Run the project.
5. Find the cursor stuck on the "@"
6. Try to add another prefix "@", select any auto complete suggestion.

Expected result:

Search suggestion when tapped should attach next to the cursor.

Actual result:

You'll find the selected suggestion text attach itself all the way at the start of the textview.