Ignore mention if the previous character is not a whitespace

Signed-off-by: Shitiz Garg <mail@dragooon.net>
Improve mentions suggestion's matching while posting

Take care of potential posts with UTF-8 based names, names with spaces. Also, cache empty results and don't query further if the names start failing.

Signed-off-by: Shitiz Garg <mail@dragooon.net>
Merge pull request #1794 from Dragooon/release-2.1

Mentions improvement