Changed activity notes to display first MEANINGFUL line.

This PR fixes displaying of notes in activity view, when a note starts with a markdown comment, like this:
```
[//]: This is a comment
```
Previous behavior: if comment is less than 150 chars, then an empty note would be displayed. Otherwise, the first 150 chars of comment would be displayed.
New behavior: 150 chars of first meaningful line will be displayed. Comments are ignored.