[Sprint/OldfashionedOwl] (fix) tag autocomplete should work with observable property

closes #1262 (closed)

To reproduce Bug in mobile app:

1. tap on poster to make a new post
2. write something, then try to tag a channel writing the first chars like @juannms
3. the autocomplete component should show a list with channels
4. taping on a channel that you want to autocomplete doesn't work